Syphilis
For example, if a person has neurosyphilis, they need antibiotics that will kill the infection in their central nervous system (their brain and major nerves). However, it is hard for penicillin G to get into the central nervous system. Because of this, a single shot of penicillin - which will usually cure early syphilis - is not enough to cure neurosyphilis. Instead, people with neurosyphilis usually need to be given large doses of penicillin for at least 10 days. Usually, they also need to get the medicine intravenously (through a needle placed into a vein) so it goes more directly to the central nervous system. If a person is allergic to penicillin, other antibiotics like ceftriaxone, doxycycline, or tetracycline can be used, but they have to be given over longer periods of time.