Penicillin G, administered parenterally, is the preferred drug for treatment of all stages of syphilis. The preparations used, the dosage, and the length of treatment depend on the stage and clinical manifestations of disease. Parenteral penicillin G is the only therapy with documented efficacy for syphilis during pregnancy. Pregnant women with syphilis in any stage who report penicillin allergy should be desensitized and treated with penicillin. The Jarisch-Herxheimer reaction is an acute febrile reaction frequently accompanied by headache, myalgia, and other symptoms that usually occurs within the first 24 hours after any therapy for syphilis. Patients should be informed about this possible adverse reaction. The Jarisch-Herxheimer reaction occurs most often among patients who have early syphilis. Antipyretics may be used, but they have not been proven to prevent this reaction.