In new guidelines released Tuesday, U.S. health officials now recommend that certain people take the antibiotic doxycycline as a morning-after pill to lower the risk of some sexually transmitted diseases (STDs). The latest recommendations only apply to gay and bisexual men and transgender women who have had an STD in the past year and are at high risk of getting infected again. While past research has shown that doxycycline works for those populations, there's not enough evidence to recommend the preventive treatment, known as doxy PEP, for all American adults, the U.S. Centers for Disease Control and Prevention noted.