Syphilis usually is treated with penicillin, administered by injection. Other antibiotics can be used for patients allergic to penicillin. A person usually can no longer transmit syphilis 24 hours after beginning therapy. Some people, however, do not respond to the usual doses of penicillin. Therefore, it is important that people being treated for syphilis have periodic blood tests to check that the infectious agent has been completely destroyed. Persons with neurosyphilis may need to be retested for up to two years after treatment. In all stages of syphilis, proper treatment will cure the disease, but in late syphilis, damage already done to body organs cannot be reversed.