Question: how long does it take for doxycycline to work for syphilis?
Answer: Regimens of doxycycline of 100 mg orally twice daily for 14 days for early syphilis and 28 days for late syphilis have been used for many years. However, studies of doxycycline for the treatment of syphilis are rare, and their results are partly contradictory (16,–21).