Acetylsalicylic acid (ASA) blocks prostaglandin synthesis. It is non-selective for the enzymes COX-1 and COX-2. Inhibition of these enzymes results in inhibition of platelet aggregation for approximately 7-10 days (average platelet life span). The acetyl group of acetylsalicylic acid binds to a serine residue of the enzyme cyclooxygenase-1 (COX-1), resulting in irreversible inhibition. This prevents the production of the prostaglandins that cause pain. This process also stops the conversion of arachidonic acid to thromboxane A2 (TXA2), which is a potent trigger for platelet aggregation. Platelet aggregation can lead to clots and harmful venous and arterial thromboembolism, resulting in conditions such as pulmonary embolism and stroke.
Absorption is generally rapid and complete following oral administration, but may vary widely depending on route of administration. 50% to 90% of a normal therapeutic concentration of salicylate (a major metabolite of acetylsalicylic acid) binds to plasma proteins, particularly albumin. Acetylsalicylic acid is hydrolyzed to salicylic acid in plasma and then further metabolized in the liver. Excretion of salicylates (metabolites of ASA) occurs primarily through the kidney.
Acetylsalicylic acid may interact with other drugs. Aspirin is known to displace a number of drugs from protein binding sites in the blood, including the antidiabetic drugs tolbutamide and chlorpropamide, warfarin, methotrexate, phenytoin, probenecid, valproic acid (as well as beta-oxidation, an important part of valproate metabolism), and other NSAIDs. This increases the plasma concentration of the aforementioned drugs, which can lead to adverse side effects.
Corticosteroids may decrease the concentration of aspirin. Ibuprofen may abolish the antiplatelet effect of aspirin, which is used for cardioprotection and stroke prevention.
The pharmacologic activity of spironolactone may be decreased by aspirin ingestion, and it is known to compete with penicillin G for renal tubular secretion. Aspirin may also inhibit the absorption of vitamin C.