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.
Ducas J, Robson HG. Cerebrospinal Fluid Penicillin Levels During Therapy for Latent Syphilis. JAMA. 1981;246(22):2583-2584. doi:10.1001/jama.1981.03320220033020