Pharmacokinetics of benzathine penicillin G: serum levels during the 28 days after intramuscular injection of 1,200,000 units.

Because of published data suggesting the inadequacy of once-every-4-weeks intramuscular injections of benzathine penicillin G for secondary rheumatic fever prevention, serum penicillin levels were determined at 1, 3, 10, 21, and 28 days after administration of 1,200,000 units of this repository penicillin. A total of 193 samples were studied. Mean serum penicillin levels remained greater than or equal to 0.02 micrograms/ml for 21 days, but by 28 days only 44% of the serum samples had detectable levels of penicillin and only 36% had levels greater than or equal to 0.02 micrograms/ml. Patients weighing more than 45 kg had significantly lower serum penicillin levels than did those who weighed less. There were similar correlations with body surface area and with age. These data indicate that a significant percentage of patients receiving benzathine penicillin G prophylaxis for prevention of recurrent attacks of rheumatic fever are not protected during the fourth week. More frequent administration of benzathine penicillin G should be considered in instances of high risk of recurrence of rheumatic fever.