A condom is a thin barrier worn during sex that physically blocks sperm from reaching an egg and prevents direct contact with infectious body fluids. When used correctly every time, condoms are about 98% effective at preventing pregnancy. They’re also one of the only contraceptive methods that simultaneously reduce the risk of sexually transmitted infections.
The Basic Mechanism
A condom works by creating a physical wall between partners during intercourse. For external (male) condoms, a thin sheath made of latex, polyurethane, or natural membrane covers the erect penis. It catches semen when ejaculation happens, keeping sperm from entering the vagina, reaching the uterus, and fertilizing an egg. It’s a straightforward concept: if sperm can’t get through, pregnancy can’t occur.
Internal (female) condoms work on the same principle but from the other side. A thin plastic pouch lines the vagina, held in place by a closed ring near the cervix and an outer ring at the vaginal opening. This creates a barrier that collects semen before it can make contact with the cervix.
Beyond blocking sperm, condoms also prevent the exchange of genital secretions between partners. This is how they reduce STI transmission. The condom keeps infected fluids, whether semen, vaginal fluid, or blood, from contacting the other person’s mucous membranes or any small breaks in the skin.
Why the Material Matters
Not all condoms offer the same level of protection, and the difference comes down to what they’re made of. Latex and polyurethane condoms are effectively impermeable to even the smallest sexually transmitted pathogens. Lab testing confirms that particles as small as HIV and hepatitis B virus cannot pass through intact latex or polyurethane. That’s significant because hepatitis B is much smaller than HIV, and neither can get through.
Natural membrane condoms (sometimes called lambskin or animal skin condoms) are a different story. They contain naturally occurring pores in the material that are large enough for viruses to slip through. These condoms can still prevent pregnancy because sperm cells are too big to fit through the pores, but they are not recommended for STI prevention.
If you have a latex allergy, polyurethane or polyisoprene condoms are safe alternatives that still block both sperm and pathogens effectively.
Protection Against STIs: What Condoms Can and Can’t Do
Condoms provide strong protection against infections transmitted through genital fluids, such as HIV, gonorrhea, and chlamydia. These infections spread through semen, vaginal secretions, or blood, and a condom keeps those fluids contained.
Protection is more limited for infections that spread through skin-to-skin contact. Herpes, syphilis, HPV, and chancroid can all be transmitted by contact with sores, ulcers, or infected skin that looks completely normal. If the infected area is on skin the condom covers, you’re protected. If the infection is on surrounding skin (the base of the penis, the scrotum, inner thighs, or vulva), the condom won’t cover it, and transmission can still happen. This is why condoms reduce the risk of these infections but don’t eliminate it entirely.
Effectiveness: Correct Use vs. Typical Use
The gap between how well condoms can work and how well they actually work in everyday life is substantial. With perfect use, about 2 out of 100 people relying solely on condoms will become pregnant over a year. With typical use, that number jumps to 13 out of 100. The difference isn’t a flaw in the condom itself. It’s human error: inconsistent use, putting it on partway through sex, or making mistakes during application.
How to Use a Condom Correctly
Most condom failures trace back to a handful of preventable mistakes. The single most common one is not pinching the reservoir tip before rolling the condom on. That small space at the tip is designed to collect semen. If air gets trapped there instead, pressure builds unevenly during thrusting and ejaculation. The condom stretches in ways it wasn’t designed to, and the result can be sudden tearing.
To put one on correctly, pinch the tip between your fingers to squeeze out any air, then roll the condom all the way down the shaft of the erect penis. If it doesn’t roll down easily, it’s probably inside out. Don’t flip it over and reuse it, because pre-ejaculate fluid may already be on the outside. Use a new one.
After ejaculation, hold the base of the condom while withdrawing so it doesn’t slip off. Remove it while the penis is still somewhat firm. Tie it off and throw it away. Never reuse a condom.
What Damages Condoms
Oil-based lubricants are one of the most well-known threats to latex condoms. Products like petroleum jelly, baby oil, coconut oil, and many lotions weaken latex and increase breakage risk. Water-based and silicone-based lubricants are safe to use with latex. If you’re using polyurethane condoms, oil-based lubricants are generally fine, but check the packaging to be sure.
Storage matters too. Heat accelerates the breakdown of latex over time. Condoms stored at high temperatures (above roughly 100°F or 38°C) degrade faster and can become brittle, sticky, or prone to tearing. A wallet, glove compartment, or back pocket exposes condoms to body heat and friction that weaken the material. Store them in a cool, dry place instead. Always check the expiration date before use, and don’t open the wrapper with your teeth or scissors, as you can nick the condom without realizing it.
Using Condoms With Other Methods
Condoms pair well with other forms of contraception. Using them alongside hormonal birth control (the pill, an IUD, or an implant) gives you both high pregnancy prevention and STI protection, something hormonal methods alone don’t offer. Spermicide can also be used with condoms for an added layer of pregnancy prevention, though spermicide alone is far less effective than condoms.
One combination to avoid: wearing two condoms at once. Using an external condom over another external condom, or an external condom with an internal condom at the same time, creates friction between the layers. This makes both more likely to tear or slip off.