The use of salicylates in rheumatic diseases has been established for over 100 years. The more recent recognition of their modification of platelet and endothelial cell function has lead to their use in other areas of medicine.

Aspirin (acetylsalicylic acid) is still the most commonly used salicylate. After oral administration as an aqueous solution aspirin is rapidly absorbed at the low pH of the stomach millieu. Less rapid absorption is observed with other formulations due to the rate limiting step of tablet disintegration — this latter factor being maximal in alkaline pH. The rate of aspirin absorption is dependent not only on the formulation but also on the rate of gastric emptying.

Aspirin absorption follows first-order kinetics with an absorption half-life ranging from 5 to 16 minutes. Hydrolysis of aspirin to salicylic acid by nonspecific esterases occurs in the liver and, to a lesser extent, the stomach so that only 68% of the dose reaches the systemic circulation as aspirin. Both aspirin and salicylic acid are bound to serum albumin (aspirin being capable of irreversibly acetylating many proteins), and both are distributed in the synovial cavity, central nervous system, and saliva.

The serum half-life of aspirin is approximately 20 minutes. The fall in aspirin concentration is associated with a rapid rise in salicylic acid concentration. Salicylic acid is renally excreted in part unchanged and the rate of elimination is influenced by urinary pH, the presence of organic acids, and the urinary flow rate. Metabolism of salicylic acid occurs through glucuronide formation (to produce salicyl acyt glucuronide and salicyl phenolic glucuronide), conjugation with glycine (to produce salicyluric acid), and oxidation to gentisic acid. The rate of formation of salicyl phenolic glucuronide and salicyluric acid are easily saturated at low salicylic acid concentrations and their formation is described by Michaelis-Menten kinetics. The other metabolic products follow first-order kinetics. The serum half-life of salicylic acid is dose-dependent; thus, the larger the dose employed, the longer it will take to reach steady-state. There is also evidence that enzyme induction of salicyluric acid formation occurs.

No significant differences exist between the pharmacokinetics of the salicylates in the elderly or in children when compared with young adults. Apart from differences in free versus albumin-bound salicylate in various disease states and physiological conditions associated with low serum albumin, pharmacokinetic parameters in patients with rheumatoid arthritis, osteoarthritis, chronic renal failure or liver disease are essentially the same. Pharmacokinetic interactions with various non-steroidal anti-inflammatory drugs do occur, but the clinical relevance of these is uncertain. Clinically important interactions may occur with heparin or oral anticoagulants, but these are due mainly to an effect on platelet function rather than on pharmacokinetic parameters.