A contraceptive pill is a daily oral medication that uses synthetic hormones to prevent pregnancy. It is one of the most widely used forms of birth control in the world, and with perfect use it is 99.7% effective. In real life, where people occasionally miss pills or take them late, effectiveness drops to about 91% over a year of use.
How the Pill Prevents Pregnancy
The pill works through several mechanisms at once. Its primary job is stopping ovulation, meaning your ovaries don’t release an egg each month. Without an egg, pregnancy can’t happen. But the pill also creates backup layers of protection: it thickens the mucus at the opening of the cervix, making it difficult for sperm to pass through, and it thins the lining of the uterus so that a fertilized egg would have a harder time implanting.
Combined Pills vs. Progestin-Only Pills
There are two main types of contraceptive pill, and they differ in what hormones they contain and how strictly you need to follow the schedule.
Combined pills contain two hormones: a synthetic estrogen (usually in doses of 10 to 35 micrograms) and a progestin. These are the most commonly prescribed type. They come in packs with 21 active pills and 7 inactive (placebo) pills, or similar variations. You get a withdrawal bleed during the placebo week that mimics a period. Timing is a bit more flexible with combined pills, though skipping pills or taking them late still reduces effectiveness.
Progestin-only pills, sometimes called minipills, contain only progestin with no estrogen. They’re an option for people who can’t take estrogen for medical reasons. The tradeoff is a much tighter dosing window: if you take a progestin-only pill more than three hours late, you need to use a backup method like condoms for at least two days.
What Happens If You Miss a Pill
Missing one combined pill (up to 48 hours late) is relatively low-risk. Take the missed pill as soon as you remember, even if that means taking two pills in one day, and continue your pack as normal. No backup contraception is needed.
Missing two or more combined pills in a row is where things get more serious. Take the most recent missed pill right away and discard any other missed ones. Use condoms or avoid sex until you’ve taken active pills for seven consecutive days. If those missed pills fell in the last week of your active pills, skip the placebo week entirely and start a new pack immediately. If you missed pills in the first week and had unprotected sex in the previous five days, emergency contraception is worth considering.
Benefits Beyond Pregnancy Prevention
Many people take the pill for reasons that have nothing to do with contraception. Combined pills regulate irregular or unpredictable periods, which is especially useful for people with polycystic ovary syndrome (PCOS). They also reduce menstrual cramping by limiting how much the uterine lining builds up each cycle.
The pill can improve symptoms of PMS and its more severe form, PMDD, likely because it prevents the hormonal rise and fall around ovulation that triggers symptoms in some people. It’s also used to manage endometriosis pain, since the progestin component limits the growth of endometrial tissue outside the uterus. Combination pills are frequently prescribed for hormonal acne as well.
Who Should Not Take Combined Pills
Combined pills are safe for most people, but the estrogen component carries a small risk of blood clots. That risk becomes unacceptable when combined with certain other factors. You should not take combined pills if you:
- Smoke and are 35 or older, especially if you smoke 15 or more cigarettes a day
- Have a history of blood clots, stroke, or heart disease
- Get migraines with aura, which already increases stroke risk on its own
- Have uncontrolled high blood pressure (160/100 or higher)
- Have a clotting disorder such as factor V Leiden or antiphospholipid syndrome
- Are in the first three weeks after giving birth, whether breastfeeding or not
- Have serious liver disease, including liver tumors or decompensated cirrhosis
Progestin-only pills avoid the estrogen-related clot risk, which is why they’re often recommended for people with one or more of these conditions.
Medications That Can Reduce Effectiveness
Certain drugs speed up how your liver breaks down the hormones in the pill, which can leave you with levels too low to prevent pregnancy. The most well-established interaction is with rifampin, an antibiotic used for tuberculosis. This one is strong enough that backup contraception is essential during treatment.
Several seizure medications also interfere with the pill, including carbamazepine, phenytoin, and topiramate. If you take any of these, a non-oral method of birth control may be more reliable.
The picture with common antibiotics is murkier. Broad-spectrum antibiotics like azithromycin, erythromycin, penicillin-type drugs, and tetracyclines may reduce pill effectiveness, though the evidence is weaker than for rifampin. The proposed mechanisms include changes to gut bacteria that affect how hormones are reabsorbed, and faster breakdown by liver enzymes. Ciprofloxacin and trimethoprim/sulfamethoxazole are considered less likely to cause problems.
Over-the-Counter Access
Most contraceptive pills still require a prescription, but that changed partially in 2023 when the FDA approved the first daily oral contraceptive for nonprescription sale in the U.S. The product, called Opill, is a progestin-only pill containing norgestrel. It’s available at drug stores, grocery stores, convenience stores, and online without needing to see a provider first. All other formulations, including all combined pills, remain prescription-only.