Comparison of the Pharmacokinetic Profiles of Soluble Aspirin and Solid Paracetamol Tablets in Fed and Fasted Volunteers

Summary

The aim of this study was to investigate the absorption of popular preparations of two common analgesics – soluble aspirin and solid paracetamol tablets. An open, randomised, crossover study design was used to compare the pharmacokinetic parameters of soluble aspirin and solid paracetamol tablets in 16 healthy, male volunteers from the University of the Witwatersrand, South Africa, in both fed and fasted states. Plasma concentrations of paracetamol, aspirin and salicylic acid were measured. It was found that the rate of absorption was significantly faster for soluble aspirin than for solid paracetamol, regardless of fed or fasting state, considering time to maximum concentration (p < 0.01), time to first quantifiable concentrations (p < 0.05) and absorption rate (p < 0.01). Absorption rate was significantly affected by food for both soluble aspirin (p = 0.028) and for solid paracetamol (p = 0.0003). Time to maximum concentration was not significantly affected by food for soluble aspirin (p = 0.17) but significantly lengthened for solid paracetamol (p = 0.0003). The extent of absorption was affected by food in terms of maximum concentration for both drugs (p = 0.0001), with a reduction of 49% in the fed state for solid paracetamol compared to 18% for soluble aspirin, the difference between the drugs being statistically significant (p = 0.0024). The overall bioavailability of soluble aspirin was unaffected by food and the bioavailability of salicylic acid was increased in the fed state, whereas that of solid paracetamol was lowered in the fed state. Greater inter-individual variation was seen in paracetamol concentrations compared with aspirin or salicylic acid levels. In conclusion, these results show that the absorption of soluble aspirin is largely unaffected by food, whereas, in the same volunteers, the absorption of solid paracetamol tablets is greatly affected. In some volunteers, maximum plasma concentrations of paracetamol following food did not reach levels previously reported to be required for effective analgesia, and this may have implications for pain relief in some individuals. The practice in some individuals of taking aspirin tablets after food to minimise potential gastric disturbance should not affect the level of analgesia.