Adult aspirin tablets are produced in standardised sizes which vary slightly from country to country, e.g. 300 mg in Britain and 325 mg in the USA. Smaller doses are based on these standards; thus 75 and 81 mg tablets are used—there is no medical significance in the slight difference.

For adults doses are generally taken four times a day for fever or arthritis, with doses near the maximal daily dose used historically for the treatment of rheumatic fever. For the prevention of myocardial infarction in someone with documented or suspected coronary artery disease, much lower doses are taken once daily.

New recommendations from the US Preventive Services Task Force (USPSTF, March, 2009) on the use of aspirin for the primary prevention of coronary heart disease encourage men aged 45–79 and women aged 55–79 to use aspirin when the potential benefit of a reduction in myocardial infarction (MI) for men or stroke for women outweighs the potential harm of an increase in gastrointestinal hemorrhage. Regular low dose (75 or 81 mg) aspirin users had a 25% lower risk of death from cardiovascular disease and a 14% lower risk of death from any cause. Low dose aspirin use was also associated with a trend toward lower risk of cardiovascular events, and lower aspirin doses (75 or 81 mg/day) may optimize efficacy and safety for patients requiring aspirin for long-term prevention.

In children with Kawasaki disease, aspirin is taken at dosages based on body weight, initially four times a day for up to two weeks and then at a lower dose once daily for a further six to eight weeks.