Condoms are great at preventing both pregnancy and STDs. If you follow the instructions and use them every time you have vaginal, anal, and oral sex, there's very little chance of pregnancy and your chance of getting or giving an STD is much lower.
How effective are condoms against pregnancy?
If you use condoms perfectly every single time you have sex, they're 98% effective at preventing pregnancy. But people aren't perfect, so in real life condoms are about 82% effective — that means about 18 out of 100 people who use condoms as their only birth control method will get pregnant each year.
The better you are about using condoms correctly every time you have sex, the better they'll work. But there's a small chance that you will get pregnant even if you always use them the right way.
How can I make condoms more effective?
The best way to make condoms work as well as possible is to use them correctly every single time you have vaginal, oral, and anal sex. That means wearing it the whole time, from start to finish. Make sure the condom is rolled on your penis the right way before there's any skin-to-skin genital contact. Read more about how to use condoms correctly.
Using condoms + another form of birth control (like the pill, IUD, or shot) is a great way to get extra pregnancy prevention AND protection against STDs. Using withdrawal ("pulling out") while also wearing a condom can help keep sperm out of the vagina and lower the risk for pregnancy.
You shouldn't use a condom worn on the penis together with a female condom. Condoms are designed to be used on their own, and doubling up won't necessarily give you extra protection. One condom used correctly is all the protection you need.