Syphilis is caused by the bacteria Treponema pallidum. Penicillin is the drug of choice for treating all stages of syphilis. For early syphilis, a single injection of 2.4 million units of benzathine penicillin G is adequate. For late syphilis, this dose may be repeated weekly for 3 weeks. Alternative drugs like tetracycline can be used in patients allergic to penicillin, except in cases of neurosyphilis, pregnancy, or HIV infection which require desensitization to penicillin first. Patients treated for syphilis require follow up testing to monitor the effectiveness of treatment.