Cerebrospinal fluid penicillin levels during therapy for latent syphilis

Cerebrospinal fluid and serum penicillin levels were determined in patients with latent syphilis. Mean serum concentration one week after the third weekly dose of 2.4 million units penicillin G benzathine was 0.32 units/mL. Concurrent administration of probenecid orally produced a mean serum penicillin level of 0.41 units/mL. Doubled penicillin doses without and with daily orally administered probenecid resulted in mean serum concentrations of 0.75 and 1.00 units/mL, respectively. Two of six patients in the last group had CSF penicillin concentrations greater than 0.03 units/mL.