context: It's happened to a lot of women -- maybe even you. You got busy and forgot to take your birth control pill. The moment you realize it can be scary, but it doesn't necessarily mean you'll get pregnant. Birth control pills aren't 100% effective at preventing pregnancy, but they come close when taken exactly as directed. Because no method of birth control is totally guaranteed to prevent pregnancy, and because it's so easy to make mistakes, it's important to understand what birth control pills do and how you can increase the odds of them being effective. Birth control pills are considered effective, but not entirely foolproof. They're about 99% effective when you take them correctly. But that's if you take them perfectly, meaning at the same time each and every day. If you don't, your odds of becoming pregnant go up to 9%. There are different types of birth control pills, including combined pills and mini-pills. No matter which kind you use, it's crucial to take them exactly as prescribed, even on the days you don't have sex. Combined pills contain two hormones, estrogen and progestin. A pack of combined pills usually has 21 to 24 days of hormones and 4 to 7 days of reminder pills. You should get your period while taking the reminder pills. You might take one pack of combined pills every month, or you can take the hormone pills continuously to delay or stop your periods. It's generally considered safe to skip or eliminate your periods, but you should discuss this option with your doctor and then follow her instructions on when to take the pill. Mini-pills packs contain only 1 hormone, progestin. If you're taking mini-pills, it's very important to take all 28 pills at the exact same time every day. If you're late taking a pill by just 3 hours, you have to use a backup method of birth control, such as a condom. The pill doesn't start working immediately. You need to take it for at least a few days before it becomes effective. That's why it's important to use a backup contraceptive, like condoms, when you first start taking it. Talk to your doctor about how long you need to use a backup method. Some recommend you use one during your entire first pack. Talk to your doctor if you just had a baby or are breastfeeding and want to take the birth control pill. You may have to wait for a certain period of time before it's safe to start on the pill. Remember, the pill doesn't protect against HIV or other sexually transmitted diseases, so you need to continue to use condoms every time you have sex, especially with new partners, to stay safe. You should take all your pills as directed, no matter what. Skipping a pill for any reason can increase your chances of becoming pregnant. If you're tempted to skip a pill because they are causing side effects, talk to your doctor, but continue taking them. Many women who experience side effects when they first start taking the pill feel better after 3 months. If you accidentally miss one pill, there's probably no need to worry. Just take it as soon as you remember and continue taking your next pill at the regular time. If it's a mini-pill and it's been more than 3 hours, use a backup method of contraception. If you're taking combined pills and miss two or more hormone pills, you should call your doctor. What you should do next will depend on what kind of pill you're on, so she can advise you. No matter what, you should use a backup method of birth control like a condom since your chances of getting pregnant are much higher after you miss two or more pills. Improper storage. Birth control pills should be stored at room temperature, away from moisture and heat, so don't keep them in your bathroom. Make sure to keep them in their original packaging so that they're protected. Other medications. Some medicines can make your birth control pill less effective. Most antibiotics are safe to take while you're on birth control pills, but one -- rifampin ( Rifadin IV) -- can stop the pill from working. Tell your doctor you're on birth control if he prescribes you rifampin. Other medicines like mood stabilizers, epilepsy medicines, and HIV drugs can also make the pill less effective. Make sure to discuss these with your doctor. Certain herbs. The supplement St. John's Wort is popular for issues like depression or insomnia, but it can reduce the amount of hormones in the pill. Talk to your doctor if you're taking this herb and consider using a backup method of birth control while you're on it.