The drug is the go-to treatment for syphilis and the only one recommended for pregnant people. But a shortage of the injectable drug has prompted some public health agencies to ration it.

Doxy-PEP can be taken a few hours after sex and is effective at preventing sexually transmitted infections. New research finds it's less effective for women but that may not be the final word.

Taken after sex, the antibiotic doxycycline can ward off some sexually transmitted illnesses. Doctors are already prescribing it and the CDC is expected to share guidance soon for how best to use it.

Syphilis cases in California have contributed to soaring national caseloads of sexually transmitted diseases. Experts point to the advent of dating apps, decreased condom use and an increase in meth.