A single 200 mg dose of Doxycycline given as post-exposure prophylaxis has been shown to provide benefit in reducing chlamydia, gonorrhea, and syphilis infections.
Link below to a CDC informational webinar on the Draft Guidelines for the Use of Doxycycline Post-Exposure Prophylaxis for Bacterial Sexually Transmitted Infection (STI) Prevention. The public comment period is now open and ends on November 6th.