How do condoms prevent STI transmission?
Male (external) condoms create a physical barrier that prevents the exchange of fluids — semen, vaginal fluid, and blood — which carry many STI pathogens. They also reduce skin-to-skin contact, which is the route of transmission for infections like herpes and HPV. Female (internal) condoms offer similar protection and also cover more of the genital area. Condoms must be used correctly and consistently to be effective — a condom that slips, breaks, or is put on after the start of sex provides incomplete protection.
Effectiveness by infection
HIV: consistent correct condom use reduces transmission risk by approximately 85% for penile-vaginal sex and is highly effective for anal sex. Chlamydia and gonorrhoea: condoms are very effective when used consistently (estimated 50–90% reduction in risk in real-world use). Syphilis: moderate protection — syphilis can be transmitted from lesions not covered by the condom. Herpes: partial protection, as HSV can be shed from areas of skin not covered. HPV: partial protection for the same reason; evidence shows condom use reduces — but does not eliminate — HPV acquisition. Hepatitis B: effective at preventing sexual transmission.
Condoms for oral sex
Dental dams and condoms used during oral sex can reduce (but not eliminate) the risk of transmitting gonorrhoea, herpes, syphilis, and HPV via the throat. In practice, these barriers are used infrequently. The risk from unprotected oral sex is lower than from anal or vaginal sex for most infections, but it is not zero — particularly for gonorrhoea, herpes, and syphilis.
Using condoms alongside other prevention methods
Condoms are most effective when used as part of a broader prevention strategy. Combining condom use with regular STI testing, HPV vaccination, hepatitis B vaccination, and PrEP (for HIV prevention) provides the most comprehensive protection. No single method is 100% effective against all infections.