Which Prevention Methods Actually Work?

Two things top the list. If you're HIV-negative, PrEP lowers your risk from sex by about 99% when taken as prescribed (now available as a daily pill, a shot every two months, or a twice-yearly injection). And if a partner is living with HIV, an undetectable viral load on treatment means they can't transmit it at all: 'Undetectable = Untransmittable' (U=U), effectively zero risk. Condoms and PEP (within 72 hours of an exposure) add further protection. PrEP is HIV-only, so pair it with condoms or doxy-PEP for other STIs.

Very well for the fluid-borne infections and pregnancy, and partially for skin-to-skin ones. Used consistently and correctly, external condoms are about 98% effective against pregnancy (around 87% with typical use) and sharply cut HIV, chlamydia, gonorrhea, and syphilis. Because they can't cover all skin, protection against herpes and HPV is partial. Condoms are the only single method that reduces both STIs and pregnancy, which is why they pair well with everything else.

Doxy-PEP is a single 200 mg dose of doxycycline taken within 72 hours after sex to prevent bacterial STIs. In trials it cut infections by about 65% on average: roughly 74–88% for chlamydia and syphilis and 55–57% for gonorrhea (lower, and affected by antibiotic resistance). It does nothing for HIV or viral STIs, and the CDC recommends it for gay and bisexual men and transgender women who've had a bacterial STI in the past year.

Yes, for two big ones. The HPV vaccine prevents about 90% of the HPV types that cause genital warts and several cancers, and the hepatitis B vaccine is over 90% effective, both work best given before exposure. There's no vaccine yet for HIV, herpes, chlamydia, or gonorrhea. Vaccination is one of the most durable forms of prevention because the protection lasts for years.

There isn't one, that's the honest answer. Each method has gaps: PrEP is HIV-only, condoms can't cover all skin, doxy-PEP is bacterial-only, vaccines cover specific viruses. The strongest protection comes from layering: condoms plus PrEP if you're at HIV risk, the HPV and hepatitis B vaccines, doxy-PEP if you qualify, and routine testing so anything is caught early. Combining a few methods covers far more than any one alone.