Daily penicillin serum concentrations following injection of 2.4 mega-units of "all purpose" penicillin

In order to investigate the reliability with which a given dose of benzathine penicillin will result in predictable ranges of penicillinaemia on any particular day during the week after injection, ambulant adult males were injected with 2 400 000 IU of "all-purpose" penicillin (600 000 IU potassium penicillin G, 600 000 IU procaine penicillin G and 1 200 000 IU of benzathine penicillin). Penicillin serum assays were performed, 24 each day, from the third to the seventh day after injection (120 assays in all).Statistical evaluation of the results showed that the means of the groups of 24 assays fell within narrow ranges, indicating that the long-acting component (benzathine penicillin) gives reliable daily ranges in a high proportion of cases. The results of a previous trial, using half the present dosage (1.2 mega-units of "all-purpose" penicillin) were compared statistically with those of the present trial. A very satisfactory degree of correlation between dosage and resulting daily serum concentration was observed-in general double the dose yielded double the daily concentration. The many factors which affect absorption rate are discussed and it is suggested that preparations which depend on an oily gel to delay absorption add an avoidable factor to the list of variables which may play an important part in producing the significant differences in serum levels commonly reported after the use of PAM preparations. The narrow ranges of penicillinaemia observed after 1.2 mega-units and 2.4 mega-units of benzathine penicillin and the degree of correlation observed in general between dose and resulting serum levels suggest that a large-scale controlled series of parallel trials should be undertaken to compare the relative long-acting qualities of PAM and benzathine penicillin.