The effect of caffeine use on a visual information processing task

Past caffeine studies have shown that differences in task performance may be due to differences in habitual caffeine consumption levels of individuals. No study has yet investigated this caffeine user effect alone without confounding it with the administration of caffeine. The present study used a visual search/detection task to test whether high caffeine users have an enhanced resource capacity and therefore show higher baseline performance than low and moderate caffeine users. A three-way interaction between caffeine user (low, moderate, and high caffeine user), automaticity (automatic and non-automatic), and task difficulty (low and high) was predicted, with user effect significant for the non-automatic portion of the task. The three-way interaction was not significant, suggesting that user effect is not dependent on resource capacity. Several interpretations, based on theoretical, empirical, methodological, and statistical viewpoints, are discussed as to the non-significant caffeine user effect.     

Caffeine and automaticity in encoding prelexical tasks: Theory and some data

The present study examined the integration of the automaticity theory and Yerkes-Dodson law using a three-factor design-Automaticity (automatic and nonautomatic behaviors), Drug (caffeine-high arousal and placebo-low arousal), and Task Difficulty (low and high levels of difficulty). A specific significant three-way interaction was predicted; that is, caffeine facilitates low difficity nonautomatic performance relative to placebo but decreases high difficulty nonautomatic performance relative to placebo, and caffeine does not affect automatic performance across levels of task difficulty relative to placebo. Three variations of the automatic/nonautomatic distinctions were used to examine its effects on the encoding and prelexical paradigm of information processing, thereby permitting a comparison of the predicted three-way interaction. The tasks were frequency monitoring (automatic)/free recall (nonautomatic) task in Experiment 1, and a variant (of task in Exp. 1) in Experiment 2 and an alternate task (vs tasks in Exps. 1 and 2) in Experiment 3. The results of Exp. 1 showed instances of the three-way interaction and thus, provides support for the proposed integration of the theory of automaticity and Yerkes-Dodson law. Overall, the results of the three experiments agreed with past findings. That, automatic performance is unaffected by empirical manipulation such as drugs and nonautomatic performance is affected by the manipulation of arousal (caffeine) in Exps. 1 and 2 and task demands (difficulty) in Exps. 2 and 3. Automatic performance, however, is not insensitive to all manipulations; Exps. 1 and 3 showed that task demands (difficulty) significantly affected it. So, there is more than one form of automaticity. In sum, the present study showed a method of comparing various forms of automaticity within a single experiment and using a pharmacological agent (drug) to test concepts of automaticity and other mechanisms of information processing.     

Effects of caffeine on performance and mood depend on the level of caffeine abstinence

Abstract Rationale. Most studies of the effects of caffeine on performance have used regular caffeine consumers who are deprived at test. Thus the reported effects of caffeine could be explained through reversal of caffeine withdrawal. Objectives. To test how preloading deprived caffeine consumers with 0, 1 or 2 mg/kg caffeine altered the subsequent ability of caffeine to modify mood and performance. Methods. Thirty moderate caffeine consumers were given a drink containing 0, 1 or 2 mg/kg caffeine at breakfast followed 60 min later by a second drink containing either 0 or 1 mg/kg caffeine. Performance on a measure of sustained attention and mood were measured before and after each drink. Results. Administration of both 1 and 2 mg/kg caffeine at breakfast decreased reaction time and 1 mg/kg caffeine also increased performance accuracy on the sustained attention (RVIP) task relative to placebo. Both breakfast doses of caffeine also improved rated mental alertness. Similarly, 1 mg/kg caffeine administered 60 min after breakfast decreased reaction time and increased rated mental alertness in the group who had not been given caffeine at breakfast. However, this second dose of caffeine had no effect on subsequent performance or mood in the two groups who had received caffeine at breakfast. Conclusions. Caffeine reliably improved performance on a sustained attention task, and increased rated mental alertness, in moderate caffeine consumers who were tested when caffeine-deprived. However, caffeine had no such effects when consumers were no longer caffeine deprived. These data are consistent with the view that reversal of caffeine withdrawal is a major component of the effects of caffeine on mood and performance. Electronic Publication     

Effects of modafinil on cognitive and meta-cognitive performance

The stimulant modafinil has proved to be an effective treatment modality for narcolepsy and related sleep disorders and is also being studied for use during sustained military operations to ameliorate the effects of fatigue due to sleep loss. However, a previous study reported that a relatively large, single dose of modafinil (300 mg), administered to already sleep-deprived individuals, caused participants to overestimate their cognitive abilities (i.e. 'overconfidence'). Because the predominant application of modafinil is in otherwise healthy, non-sleep-deprived individuals, the present study investigated the generality of modafinil-induced overconfidence in a group of 18 healthy, non sleep-deprived adults. The design involved a double-blind, placebo controlled, fully within-subjects manipulation of placebo and modafinil (4 mg/kg: approximately 300 mg, on average) over three 50-min cognitive testing sessions (i.e. before drug ingestion, and at 90 and 180 min after drug ingestion). The cognitive task battery included subjective assessments of mood, fatigue, affect, vigor and motivation, and cognitive assessments of serial reaction time, logical reasoning, visual comparison, mental addition and vigilance. In addition, trial-by-trial confidence judgements were obtained for two of the cognitive tasks and more global, task level assessments of performance were obtained for four of the cognitive tasks. Relative to placebo, modafinil improved fatigue levels, motivation, reaction time and vigilance. In terms of self-assessments of cognitive performance, both the placebo and modafinil conditions were 'well calibrated' on trial-by-trial confidence judgements, showing neither marked over- nor under-confidence. Of note, the modafinil condition displayed a non-significant tendency towards 'overconfidence' for task-level assessments of performance. The present findings highlight the need for continued research on the many complex interactions involving fatigue states, occasional versus long-term stimulant use, and subjective assessments of fatigue and cognitive performance.     

Psychomotor performance: investigating the dose-response relationship for caffeine and theophylline in elderly volunteers

Objective: The aim of this study was to investigate the dose-response relationship for psychomotor performance, caffeine and theophylline in healthy elderly volunteers. Methods: In a randomized, double-blind, placebo-controlled, six-period cross-over study we compared the effect of three doses of theophylline (predicted peak concentrations of 3, 6 mg · l⁻¹ and 12 mg · l⁻¹), two doses of caffeine (predicted peak concentrations of 4.5 mg · l⁻¹ and 9 mg · l⁻¹) and placebo on ten healthy elderly volunteers. Psychomotor performance was measured using a continuous attention task, symbol digit substitution test and choice reaction time. Subjective effects were assessed using visual analogue scales. Following drug administration, subjects received the test battery at 30-min intervals, up to 150 min. Maximum and mean effects from baseline on each variable were included in the analysis. Results: Significant improvement on the continuous attention task was seen at the lowest concentration of caffeine and theophylline used, while at higher concentrations there was a non-significant trend towards placebo scores. There was little effect of either drug on the subjective effects measured by visual analogue scales. Conclusion: Caffeine and theophylline increase psychomotor performance measures of attention at low plasma concentrations in healthy elderly volunteers. This effect is not increased by higher drug concentrations and there is trend towards a return to placebo scores. The lack of effect of both caffeine and theophylline on subjective measures is consistent with previous studies of caffeine in the elderly. Received: 11 December 1997 / Accepted in revised form: 18 March 1998     

Effects of a low dose of caffeine given in different drinks on mood and performance

An experiment was carried out to examine the effects of 40 mg of caffeine given in different drinks (coffee, water, tea, cola) on mood and performance. One hundred and forty‐four volunteers were randomly assigned to one of the groups formed by combining the caffeine/placebo and drinks conditions. Following a baseline session measuring mood and different aspects of performance, the volunteers were given their drink and then carried out another test session 1 h later. Administration of the caffeine/placebo was double‐blind. The results showed that those given caffeine reported greater alertness and anxiety at the end of the test session, as well as improved performance on choice reaction time tasks involving focused attention and categoric search, a semantic memory task and a delayed recognition memory task. The effect of the caffeine was not modified by the nature of the drink in which it was given. Overall, these results show that a dose of caffeine typical of the level found in commercial products can improve alertness and performance efficiency. Copyright © 1999 John Wiley & Sons, Ltd.     

The effects of caffeine, impulsivity and time of day on performance, mood and cardiovascular function

Two experiments were carried out to examine the effects of caffeine on performance, mood and cardiovascular function. The results showed that the effects of caffeine depended on the dose, time of administration, the function being examined and the impulsivity of the subject. Changes in blood pressure were only observed when a high dose (3 mg/kg) was used. The effects of this dose on performance depended on the impulsivity of the subject, with high impulsives performing worse in the de-caffeinated condition but getting a greater benefit from the caffeine. The high dose of caffeine also removed the post-lunch dip in sustained attention. The second experiment, which used a lower dose of caffeine (~60 mg), failed to demonstrate any caffeine x impulsivity or caffeine x time of day effects on performance. However, caffeine improved performance on a logical reasoning task and caffeine x time of day x impulsivity effects were found in analyses of visual search tasks. The mood data also support the view that the effects of caffeine depend on a combination of factors similar to those outlined for performance.     

Subjective, behavioral, and physiological effects of acute caffeine in light, nondependent caffeine users

Rationale Caffeine produces mild psychostimulant effects that are thought to underlie its widespread use. However, the direct effects of caffeine are difficult to evaluate in regular users of caffeine because of tolerance and withdrawal. Indeed, some researchers hypothesize that the psychostimulant effects of caffeine are due largely to the reversal of withdrawal and question whether there are direct effects of caffeine consumption upon mood, alertness, or mental performance in nondependent individuals.Objective This study investigated the physiological, subjective, and behavioral effects of 0, 50, 150, and 450 mg caffeine in 102 light, nondependent caffeine users.Methods Using a within-subjects design, subjects participated in four experimental sessions, in which they received each of the four drug conditions in random order under double blind conditions. Participants completed subjective effects questionnaires and vital signs were measured before and at repeated time points after drug administration. Forty minutes after the capsules were ingested, subjects completed behavioral tasks that included tests of sustained attention, short-term memory, psychomotor performance, and behavioral inhibition.Results Caffeine significantly increased blood pressure, and produced feelings of arousal, positive mood, and high. Caffeine increased the number of hits and decreased reaction times in a vigilance task, but impaired performance on a memory task.Conclusion We confirm that acute doses of caffeine, at levels typically found in a cup of coffee, produce stimulant-like subjective effects and enhance performance in light, nondependent caffeine users. These findings support the idea that the drug has psychoactive effects even in the absence of withdrawal.     

Effects of caffeine,time of day and user history on study-related performance

The individual and interactive effects of caffeine, time of day and history of caffeine consumption on several study-related tasks were investigated in 25 subjects (6 males, 19 females). Performance was measured on short term memory (STM), mental arithmetic (MA), reading comprehension, serial search (SS) and verbal reasoning (VR). Subjects attended eight experimental sessions, at four times of day (0100, 0700, 1300 and 1900 hours), after ingesting caffeine (4 mg/kg) or placebo. Subjects were assigned to a low, moderate or high user group on the basis of a caffeine consumption questionnaire. Reading comprehension was affected by time of day, while caffeine improved performance on all mental speed-related tasks. High caffeine users performed more poorly than other groups on the verbal reasoning task. Several interactions between the three independent variables were observed on a number of tasks, supporting the contention that different processes underlying various types of cognitive performance are differentially, and often jointly, affected by caffeine, time of day and user history. Implications of caffeine usage on academic performance were discussed.     

Tracking the cognitive pharmacodynamics of psychoactive substances with combinations of behavioral and neurophysiological measures.

Many common pharmacological treatments have effects on cognitive ability. Psychometric task batteries used to characterize such effects do not provide direct information about treatment-related changes in brain function. Since overt task performance reflects motivation and effort as well as ability, behavioral measures alone may overestimate or underestimate the impact of a pharmacological intervention on brain function. Here we present a method that combines behavioral and neurophysiological measures in an attempt to detect the psychoactive effects of pharmacological treatments with greater sensitivity than that provided by behavioral measures alone. Initial application of the method is made to the data from a double blind, placebo-controlled, crossover study in which caffeine, diphenhydramine, and alcohol were used to alter the mental state of 16 healthy subjects at rest and while they performed low load and high load versions of a working memory task. For each intervention, more sensitive detection of drug or alcohol effects over a four hour period was obtained when EEG variables were included in multivariate analyses than when only behavioral variables were used. These initial results suggest that it can be useful to incorporate neurophysiological measures of brain activity into inferences concerning the acute impact of drugs on mental function, and demonstrate the feasibility of using multivariate combinations of behavioral and neurophysiological measures to sensitively characterize the pharmacodynamics of drug-induced changes in cognition.     

Subjective effects of caffeine among introverts and extraverts in the morning and evening.

In previous studies of psychomotor performance, the stimulant effects of caffeine differed by personality characteristics. For example, caffeine improved the task performance of extraverts but overaroused introverts and thus impaired their performance. The present study compared the effects of caffeine on subjective arousal among introverts and extraverts. Seventeen introverts and 19 extraverts drank coffee that contained doses of 0, 2, and 4 mg/kg caffeine during morning and evening sessions in a within-subjects, randomized, double-blind, crossover design. At 30-min intervals for 180 min after drinking, participants completed the Profile of Mood States, a battery of self-report visual analog scales, and the Digit Symbol Substitution Test (DSST). Caffeine effects on mood and task performance did not significantly interact with extraversion, except for nonsignificant trends for caffeine to increase happiness and vigor more among extraverts than introverts. No 3-way interactions of group, time, and dose were found on any scales or on the DSST. Results do not support the hypothesis that caffeine differentially affects extraverts and introverts, particularly at different times of the day.     

Cognitive and mood improvements of caffeine in habitual consumers and habitual non-consumers of caffeine

Rationale The cognitive and mood effects of caffeine are well documented. However, the majority of studies in this area involve caffeine-deprived, habitual caffeine users. It is therefore unclear whether any beneficial findings are due to the positive effects of caffeine or to the alleviation of caffeine withdrawal.Objectives The present placebo-controlled, double-blind, balanced crossover study investigated the acute cognitive and mood effects of caffeine in habitual users and habitual non-users of caffeine.Method Following overnight caffeine withdrawal, 24 habitual caffeine consumers (mean=217 mg/day) and 24 habitual non-consumers (20 mg/day) received a 150 ml drink containing either 75 or 150 mg of caffeine or a matching placebo, at intervals of 48 h. Cognitive and mood assessments were undertaken at baseline and 30 min post-drink. These included the Cognitive Drug Research computerised test battery, two serial subtraction tasks, a sentence verification task and subjective visual analogue mood scales.Results There were no baseline differences between the groups mood or performance. Following caffeine, there were significant improvements in simple reaction time, digit vigilance reaction time, numeric working memory reaction time and sentence verification accuracy, irrespective of group. Self-rated mental fatigue was reduced and ratings of alertness were significantly improved by caffeine independent of group. There were also group effects for rapid visual information processing false alarms and spatial memory accuracy with habitual consumers outperforming non-consumers. There was a single significant interaction of group and treatment effects on jittery ratings. Separate analyses of each groups responses to caffeine revealed overlapping but differential responses to caffeine. Caffeine tended to benefit consumers mood more while improving performance more in the non-consumers.Conclusions These results do not support a withdrawal alleviation model. Differences in the patterns of responses to caffeine by habitual consumers and habitual non-consumers may go some way to explaining why some individuals become caffeine consumers.     

The effects of caffeine and expectancy on attention and memory

The present study contrasted caffeine's effects on individuals who expect caffeine to stimulate them and those who do not. Secondly, whether a message that caffeine rather than placebo was administered would also affect these two groups of subjects differently was investigated. The study was conducted single-blind in a 2x2x2 mixed design. The between subjects factor was whether they expected caffeine to stimulate them (E+) or not (E-) according to their self reports obtained before the experiment began. The within subjects factors were message (told caffeine vs told placebo) and beverage type (given caffeine vs placebo). Sixteen subjects in each group (n=32) performed on signal detection, memory scanning and delayed free recall tasks following ingestion of either caffeinated or decaffeinated coffee on two sessions each, a total of four experimental sessions. On each session, subjects were given a message regarding their drink (told caffeine vs told placebo). However, on two sessions there was a mismatch between the message and drink given. For signal detection, performance under caffeine was better than placebo in the E+ but not the E- group. However, subjects in the E+ group did not benefit more than the E- group in either message condition. On memory scanning, detections and false alarms did not differ for either beverage, nor was there a differential finding in the E+ and E- groups. However, reaction time under caffeine condition was shorter. No effects of message were found. Caffeine and message also did not have any effect on performance on the delayed free recall task. The hypothesis that caffeine and message would affect E+ and E- subjects differentially was partly supported.     

Methamphetamine and diphenhydramine effects on the rate of cognitive processing

Three cognitive tasks in which performance depends primarily on the rate of cognitive processing were given to 24 male subjects before and after oral doses of methamphetamine (10 mg), diphenhydramine hydrochloride (100 mg), and placebo. Each subject was tested on Monday, Wednesday, and Friday of one week with drug orders balanced across subjects. Compared with placebo and diphenhydramine, methamphetamine increased the rate at which a visual display was scanned for a target stimulus. Methamphetamine affected neither a time-production task nor a divided attention task that required the subject to perform two cognitive tasks in a limited amount of time. This suggests that methamphetamine can increase cognitive processing speed on tasks involving familiar cognitive operations but that an increase is not likely in tasks involving more complicated decision processes. Compared with placebo and methamphetamine, diphenhydramine caused subjects no experience geophysical time as passing more slowly, but the drug had no significant effects on the visual search or divided-attention tasks. This suggests that time perception is more likely to be altered by diphenhydramine than is performance on tasks requiring short periods of rapid cognitive processing.     

Influence of Grapefruit Juice on Caffeine Pharmacokinetics and Pharmacodynamics

The influence of grapefruit juice (GFJ) on caffeine's metabolism and the hemodynamic effects of this potential food interaction were studied in 10 normotensive volunteers. In this crossover study, caffeine (3.3 mg/kg) and water or caffeine and GFJ were given to participants. Nine serum caffeine concentrations were determined within 24 hours of each phase. In another phase of this study, caffeine was given with multiple GFJ doses to 6 of the 10 participants. Ambulatory blood pressure (BP) monitors were used for 12 hours to assess treatment hemodynamic effects. The mean area under the serum caffeine concentration-time curve (AUC_0–∞) values ± SD for the caffeine with water group, caffeine with GFJ group, and caffeine with multiple GFJ group were 47.0 ± 10.8, 48.7 ± 15.2, and 49.6 ± 7.0 μg/ml · hr, respectively (NS). There was no significant difference on the ambulatory systolic BP, diastolic BP, percentage of the time with a diastolic BP greater than 90 mm Hg, or heart rate area under the effect curves. We conclude that grapefruit juice had no effect on caffeine pharmacokinetics or hemodynamic effects.     

Effects of low doses of caffeine on cognitive performance, mood and thirst in low and higher caffeine consumers

RATIONALE: Caffeine is present in many widely consumed drinks and some foods. In the fairly extensive literature on the psychostimulant effects of caffeine, there are few dose-response studies and even fewer studies of the effects of doses of caffeine lower than 50 mg (the range of the amounts of caffeine contained in, for example, a typical serving of tea or cola). OBJECTIVE: This study measured the effects of 0, 12.5, 25, 50 and 100 mg caffeine on cognitive performance, mood and thirst in adults with low and moderate to high habitual caffeine intakes. METHODS: This was a double-blind, within-subjects study. Following overnight caffeine abstinence, participants (n=23) completed a test battery once before and three times after placebo or caffeine administration. The test battery consisted of two performance tests, a long duration simple reaction time task and a rapid visual information processing task, and a mood questionnaire (including also an item on thirst). RESULTS: Effects on performance and mood confirmed a psychostimulant action of caffeine. All doses of caffeine significantly affected cognitive performance, and the dose-response relationships for these effects were rather flat. The effects on performance were more marked in individuals with a higher level of habitual caffeine intake, whereas caffeine increased thirst only in low caffeine consumers. CONCLUSIONS: After overnight caffeine abstinence, caffeine can significantly affect cognitive performance, mood and thirst at doses within and even lower than the range of amounts of caffeine contained in a single serving of popular caffeine-containing drinks. Regular caffeine consumers appear to show substantial tolerance to the thirst-increasing but not to the performance and mood effects of caffeine.     

Caffeine Moderately Antagonizes the Effects of Triazolam and Zopiclone on the Psychomotor Performance of Healthy Subjectsf

Abstract: To determine whether caffeine antagonizes the decremental effects of triazolam and zopiclone on human performance, oral single doses of 0.250 mg triazolam, 7.5 mg zopiclone, or respective placebos, with and without 300 mg caffeine, were given to parallel groups of student volunteers in two double-blind studies. Objective tests and subjective visual analogue ratings were done at baseline and 30 min. and 90 min. after the intake. In Study I, triazolam produced drowsiness at 30 min. but did not differ from the placebo in other tests. Caffeine induced alerting effects in various tests and differed from triazolam in some (digit substitution, drowsiness, calmness, mental slowness) but not all variables measured. Caffeine and triazolam were interpreted as being antagonists. In Study II, zopiclone impaired digit substitution and flicker fusion, produced exophoria and lowered systolic blood pressure. Caffeine differed from zopiclone in several test functions, but it also differed from caffeine + zopiclone whereas zopiclone differed from caffeine + zopiclone only in two tests (Maddox wing, systolic blood pressure). Thus, zopiclone counteracted the effects of caffeine more easily than caffeine counteracted the decremental effects of zopiclone. We conclude that triazolam may not differ importantly from diazepam as regards their antagonism towards caffeine, whereas further research on the antagonism between zopiclone and caffeine needs to be done.     

Caffeine moderately antagonizes the effects of triazolam and zopiclone on the psychomotor performance of healthy subjects.

To determine whether caffeine antagonizes the decremental effects of triazolam and zopiclone on human performance, oral single doses of 0.250 mg triazolam, 7.5 mg zopiclone, or respective placebos, with and without 300 mg caffeine, were given to parallel groups of student volunteers in two double-blind studies. Objective tests and subjective visual analogue ratings were done at baseline and 30 min. and 90 min. after the intake. In Study I, triazolam produced drowsiness at 30 min. but did not differ from the placebo in other tests. Caffeine induced alerting effects in various tests and differed from triazolam in some (digit substitution, drowsiness, calmness, mental slowness) but not all variables measured. Caffeine and triazolam were interpreted as being antagonists. In Study II, zopiclone impaired digit substitution and flicker fusion, produced exophoria and lowered systolic blood pressure. Caffeine differed from zopiclone in several test functions, but it also differed from caffeine + zopiclone whereas zopiclone differed from caffeine + zopiclone only in two tests (Maddox wing, systolic blood pressure). Thus, zopiclone counteracted the effects of caffeine more easily than caffeine counteracted the decremental effects of zopiclone. We conclude that triazolam may not differ importantly from diazepam as regards their antagonism towards caffeine, whereas further research on the antagonism between zopiclone and caffeine needs to be done.     

Caffeine attenuates scopolamine-induced memory impairment in humans

Caffeine consumption can be beneficial for cognitive functioning. Although caffeine is widely recognized as a mild CNS stimulant drug, the most important consequence of its adenosine antagonism is cholinergic stimulation, which might lead to improvement of higher cognitive functions, particularly memory. In this study, the scopolamine model of amnesia was used to test the cholinergic effects of caffeine, administered as three cups of coffee. Subjects were 16 healthy volunteers who received 250 mg caffeine and 2 mg nicotine separately, in a placebo-controlled double-blind cross-over design. Compared to placebo, nicotine attenuated the scopolamine-induced impairment of storage in short-term memory and attenuated the scopolamine-induced slowing of speed of short-term memory scanning. Nicotine also attenuated the scopolamine-induced slowing of reaction time in a response competition task. Caffeine attenuated the scopolamine-induced impairment of free recall from short- and long-term memory, quality and speed of retrieval from long-term memory in a word learning task, and other cognitive and non-cognitive measures, such as perceptual sensitivity in visual search, reading speed, and rate of finger-tapping. On the basis of these results it was concluded that caffeine possesses cholinergic cognition enhancing properties. Caffeine could be used as a control drug in studies using the scopolamine paradigm and possibly also in other experimental studies of cognitive enhancers, as the effects of a newly developed cognition enhancing drug should at least be superior to the effects of three cups of coffee.     

Time for tea: mood, blood pressure and cognitive performance effects of caffeine and theanine administered alone and together

Rationale Although both contain behaviourally significant concentrations of caffeine, tea is commonly perceived to be a less stimulating drink than coffee. At least part of the explanation for this may be that theanine, which is present in tea but not coffee, has relaxing effects. There is also some evidence that theanine affects cognitive performance, and it has been found to reduce blood pressure in hypertensive rats. Objectives To study the subjective, behavioural and blood pressure effects of theanine and caffeine administered alone and together, in doses relevant to the daily tea consumption of regular tea drinkers. Materials and methods In a randomised, double-blind, placebo-controlled study, healthy adult participants (n = 48) received either 250-mg caffeine, 200-mg theanine, both or neither of these. They completed ratings of mood, including anxiety, and alertness, and had their blood pressure measured before and starting 40 min after drug administration. Anxiety was also assessed using a visual probe task. Results Caffeine increased self-rated alertness and jitteriness and blood pressure. Theanine antagonised the effect of caffeine on blood pressure but did not significantly affect jitteriness, alertness or other aspects of mood. Theanine also slowed overall reaction time on the visual probe task. Conclusions Theanine is a physiologically and behaviourally active compound and, while it is unclear how its effects might explain perceived differences between tea and coffee, evidence suggests that it may be useful for reducing raised blood pressure.