The main common causes of rash flare-ups after a syphilis shot include Hirschsprung’s reaction and allergic reaction. Patients with syphilis rash flare-ups after a shot should go to the hospital in time to find out the cause of the disease. Syphilis is a sexually transmitted disease caused by infection with the syphilis spirochete, which can invade organs throughout the body and cause multiple organ damage. Syphilis injection generally refers to antibiotics for the treatment of syphilis, mainly benzylpenicillin, but also may be procaine aqueous penicillin and other components. The main reasons for the rash attack after a syphilis shot include Hirschsprung’s reaction and allergic reaction. 1. Hirschsprung’s reaction: Hirschsprung’s reaction usually refers to the aggravation of spirochete infections after the application of penicillin treatment, usually due to the antibiotic to make the spirochete a large number of cleavage, thus releasing a large number of cleavage proteins, which causes the human body to produce a perverse reaction, the symptoms of the rash attack can occur. 2. Allergic reaction: the main component of the syphilis injection is penicillin, when the patient is allergic to penicillin, there will be a metamorphic allergic reaction of the body, which will lead to the symptoms of rash attack. Syphilis after a shot of rash attack patients should go to the hospital in time, under the guidance of the doctor for regular treatment.