For ocular syphilis, otic syphilis, or neurosyphilis, one of the following is recommended: Aqueous penicillin 3 to 4 million units IV every 4 hours (penetrates the central nervous system best but may be impractical) Procaine penicillin G 2.4 million units IM once a day plus 500 mg probenecid orally 4 times a day Both medications are given for 10 to 14 days, followed by benzathine penicillin 2.4 million units IM once a week for 1 to 3 weeks after completion of these neurosyphilis treatment regimens to provide a total duration of therapy comparable to that for late latent syphilis.