It appears that syphilis treatment is followed by what is called Herxheimer reaction (chills,
joint pain, body ache, fever, headache, myalgia (muscle
pain), and exacerbation of skin lesions). [source Wiki) My Doc. did not warm me about the reaction when I asked about side effects. After experiencing those reaction, I called the hospital and the Doc I talked said this is a common reaction to syphilis treatment.