LIFE IN THE 21ST CENTURY: CDC considers common antibiotic after unprotected sex to prevent STIs. “A draft recommendation from the U.S. Centers for Disease Control and Prevention could advise doctors to prescribe a strong antibiotic, after unprotected sex, to prevent sexually transmitted infections including syphilis, gonorrhea and chlamydia. Doxycycline, which could be used as a morning-after pill to prevent STI’s, would only be recommended for men who have sex with men and transgender women, according to the proposed guidelines announced Monday by the CDC.”