Plasma levels of aspirin following effervescent and enteric coated tablets, and their effect on platelet function.

Single doses of effervescent tablets (1200 mg) and enteric coated (EC) tablets (1300 mg and 650 mg) of acetylsalicylic acid (aspirin, ASA) were given to healthy volunteers in random order. Plasma ASA and salicylic acid (SA) levels were measured and concurrent in vitro measurements of the volunteers' platelet aggregation were carried out. The effervescent preparation resulted in peak ASA concentrations of 17-40 mg/l, achieved 20 to 30 min after a 1200 mg dose, whereas peak ASA levels of 0.01-0.37 mg/l were observed 4-6 h after a 650 mg dose of the EC preparation. With all the aggregating agents that were added to the test system maximum inhibition of platelet aggregation (about 50% of pre dose levels) was seen 1.0 h after the effervescent ASA dose, and persisted to at least 24 h, but with the EC preparation not until 24 h, at which time the degree of inhibition was also about 50% of pre-dose levels. A 1.0 g dose of sodium salicylate had no effect on in vitro platelet function. It was concluded that mean plasma levels of ASA of less than 0.25 mg/l are sufficient to depress aggregation by approximately 50%. A low dose of ASA taken daily either as effervescent ASA or EC ASA, significantly inhibits platelet aggregation and so may reduce the risk of ischaemic episodes in susceptible patients.