DoxyPEP means taking the antibiotic doxycycline after sex, to prevent getting an STI. 200mg (usually 2x100mg tablets) of doxycycline are taken ideally within 24 hours but no later than 72 hours after condomless oral or anal sex (research on vaginal sex is ongoing).
Research has shown efficacy against chlamydia was 88% and against syphilis 87% in HIV-negative people, and 77% and 74% in people with HIV. But efficacy against gonorrhoea was only 55% in people without HIV and 57% in people with HIV.
It is like a morning-after pill but for STIs. You can take doxycycline as often as every day when you are having condomless sex but don’t take more than 200 mg every 24 hours.