A condom is a thin barrier worn during sex to prevent pregnancy and reduce the risk of sexually transmitted infections (STIs). It works by physically blocking the exchange of bodily fluids between partners. Condoms are one of the only contraceptive methods that protect against both pregnancy and STIs at the same time, which is why they remain a cornerstone of sexual health worldwide.
Pregnancy Prevention
The most common reason people use condoms is to prevent pregnancy. An external condom (worn on the penis) is about 87% effective with typical, real-world use. That means roughly 13 out of 100 people relying on condoms as their only method will experience an unintended pregnancy over the course of a year. With perfect use, meaning the condom is put on correctly every single time and never slips or breaks, effectiveness climbs to around 97%.
The gap between typical and perfect use comes down to human error: putting a condom on too late, using the wrong lubricant, not leaving space at the tip, or reusing one. These mistakes are common enough to matter statistically, but they’re also preventable with a little practice and attention.
Internal condoms, sometimes called female condoms, serve the same purpose. They’re inserted into the vagina before sex and are about 75% to 82% effective with normal use, rising to 95% with perfect use. They give the receptive partner direct control over barrier protection.
Protection Against STIs
Condoms are highly effective at blocking infections transmitted through genital fluids, including HIV, gonorrhea, and chlamydia. Lab testing confirms that latex condoms form a barrier against even the smallest known STI pathogens. In real-world studies of couples where one partner had HIV and the other did not, consistent condom use reduced transmission dramatically. In one study, none of 123 partners who always used condoms became infected, compared to 10% of those who used them inconsistently.
That level of protection applies most strongly to infections carried in semen, vaginal fluid, or pre-ejaculate. For these fluid-borne STIs, condoms are one of the most reliable tools available outside of abstinence.
Infections Spread by Skin Contact
Condoms are less effective against STIs that spread through skin-to-skin contact, such as herpes (HSV), human papillomavirus (HPV), and syphilis. These infections can live on skin that the condom doesn’t cover, like the outer labia, the base of the penis, or the upper thigh. If the infected area happens to be covered by the condom, you get solid protection. If it’s not, transmission can still occur.
That said, condoms still reduce the overall risk of these infections. They just can’t eliminate it the way they can for something like chlamydia, where the pathogen is entirely fluid-borne.
Types of Condom Materials
Not all condoms are made from the same material, and the differences affect comfort, sensation, allergy concerns, and lubricant compatibility.
- Latex is the most widely available and well-studied material. It’s stretchy, effective against both pregnancy and STIs, and compatible with water-based or silicone-based lubricants. Oil-based products (petroleum jelly, lotion, coconut oil) break down latex in as little as 60 seconds, significantly increasing the chance of breakage.
- Polyurethane is thinner than latex and conducts heat well, which some people say makes sex feel more natural. It’s a good option for anyone with a latex allergy. Use it with water-based or silicone-based lube.
- Polyisoprene is a synthetic rubber that stretches like latex but won’t trigger latex allergies. It also requires water-based or silicone-based lubricants.
- Lambskin (natural membrane) offers a different feel and is compatible with oil-based lubricants. However, the pores in natural membrane are large enough to let viruses pass through, so lambskin condoms prevent pregnancy but do not protect against STIs.
- Nitrile is the material used in most current internal condoms. It warms to body temperature quickly and works with water-based, silicone-based, or oil-based lubricants.
How to Get the Most Protection
The effectiveness numbers only hold up when condoms are used correctly and consistently. “Consistently” means every time, from start to finish. “Correctly” means a few specific things: checking the expiration date on the wrapper before use, pinching the tip to leave a small reservoir for semen, rolling it all the way down, and using only compatible lubricants.
Storage matters too. Heat, direct sunlight, and friction (like sitting in a wallet for months) can weaken the material before you ever open the package. A condom stored in a cool, dry place and used well before its expiration date is far more reliable than one that’s been through rough treatment.
If a condom breaks or slips during sex, the protection is gone for that encounter. Emergency contraception can still reduce pregnancy risk after the fact, but there’s no equivalent backup for STI exposure.
Using Condoms With Other Methods
Many people pair condoms with a hormonal contraceptive like the pill, an IUD, or an implant. The hormonal method handles pregnancy prevention with very high reliability, while the condom adds STI protection that no hormonal method provides. This combination is sometimes called “dual protection” and is particularly useful in newer relationships or when either partner’s STI status is unknown.
Condoms can also be paired with other barrier methods, but doubling up on external condoms (wearing two at once) actually increases friction and makes breakage more likely. One condom at a time is the correct approach.
Less Common Uses
Outside of sexual health, condoms have a handful of practical applications. In medical settings, condoms are sometimes used to cover ultrasound probes during transvaginal or transrectal examinations. Studies from the early 1990s found that condoms were actually more effective as probe covers than some commercially manufactured alternatives, and they continue to be used in clinical practice for this purpose.
Condoms are also used during oral sex on a penis to reduce STI transmission, and dental dams (or a condom cut open and laid flat) serve a similar role during oral sex on a vulva or anus.