An old drug offers a new way to stop STIs

Scientists have shown that the antibiotic doxycycline can ward off illnesses like chlamydia, gonorrhea and syphilis. It's a prevention tool called doxy-PEP. (MirageC/Getty Images)

A promising line of attack against sexually transmitted infections puts a cheap and widely available medication to a new use.

The treatment – a form of post-exposure prophylaxis, or PEP – is a dose of the antibiotic doxycycline taken in the hours immediately after sex which works to extinguish an STI before it leads to symptoms or spreads to others.