An oral contraceptive is a pill taken daily to prevent pregnancy. Commonly called “the pill,” it uses synthetic hormones to stop ovulation, thicken cervical mucus, or both. With perfect use, oral contraceptives are 99.7% effective. In real-world typical use, about 9 out of 100 women become pregnant in the first year, mostly due to missed pills or inconsistent timing.
Two Types of Birth Control Pills
There are two main categories. Combined oral contraceptives (COCs) contain low doses of two hormones: a synthetic estrogen and a progestin. This is what most people mean when they say “the pill,” and it’s the more widely prescribed type. The second category is the progestin-only pill, sometimes called the “mini-pill,” which contains just one hormone and no estrogen.
The distinction matters because the two types work slightly differently, carry different risks, and suit different people. If you smoke and are over 35, have a history of blood clots, or are breastfeeding, the combined pill is off the table, but a progestin-only option may still be safe.
How the Pill Prevents Pregnancy
Combined pills work on multiple fronts. They suppress the hormonal signals from the brain that trigger ovulation, so no egg is released. They also thicken cervical mucus, creating a barrier that sperm can’t easily pass through. On top of that, they thin the uterine lining, making it less hospitable if an egg were somehow fertilized.
Progestin-only pills rely primarily on thickening cervical mucus. They do suppress ovulation in some cycles, but that isn’t their main mechanism. This is one reason timing matters more with the mini-pill: it needs to be taken within the same narrow window each day to keep mucus consistently thick enough to block sperm.
Uses Beyond Pregnancy Prevention
Millions of people take oral contraceptives for reasons that have nothing to do with preventing pregnancy. The pill is one of the most common treatments for polycystic ovary syndrome (PCOS), where international guidelines recommend combined pills to manage irregular periods and symptoms like excess hair growth and acne. By raising levels of a protein that binds testosterone in the blood, combined pills reduce the amount of free testosterone circulating in the body, which is what drives those symptoms.
Certain progestin formulations also help with premenstrual mood disturbances, bloating, and the minor weight gain some people experience on other pill types. Some work against the water and salt retention that estrogen promotes, which can slightly lower blood pressure and reduce that puffy, bloated feeling before a period.
The pill is also frequently prescribed for painful periods, endometriosis, and heavy menstrual bleeding. For people who simply want predictable, lighter periods, it delivers that reliably.
Who Should Not Take Combined Pills
Combined oral contraceptives carry real cardiovascular risks for certain people. The FDA lists several groups who should not use them:
- Smokers over age 35. The risk of serious cardiovascular events rises sharply with age and the number of cigarettes smoked.
- People with a history of blood clots, including deep vein thrombosis or pulmonary embolism.
- People with certain heart conditions, including coronary artery disease, some valve disorders, or uncontrolled high blood pressure.
- People who get migraines with aura, or any migraines if over 35.
- Those with a current or past breast cancer diagnosis.
- People with active liver disease or liver tumors.
- People with diabetes over 35, or diabetes with vascular complications or a duration longer than 20 years.
After giving birth, combined pills should not be started until at least four weeks postpartum because of a temporarily elevated clot risk. The same waiting period applies after a second-trimester miscarriage or abortion.
Blood Clot Risk in Context
The link between combined pills and blood clots is well established. Women who use oral contraceptives have roughly 3 to 5 times the risk of venous thromboembolism compared to women who have never used them, according to a large UK Biobank study of 240,000 women. The risk is highest during the first two years of use and drops after that.
That sounds alarming as a multiplier, but the baseline risk in young, healthy women is very low. A 3 to 5 fold increase of a small number is still a small number for most people. The risk becomes meaningful when other factors stack on top of it: smoking, obesity, inherited clotting disorders, prolonged immobility, or major surgery. If you have any of those factors, it’s worth a conversation about whether the combined pill is the right choice or whether a progestin-only option is safer.
What to Do When You Miss a Pill
Missing pills is the main reason the gap between perfect-use and typical-use effectiveness is so large. The CDC provides clear guidance for combined pills. If you’re less than 48 hours late (one missed pill), take it as soon as you remember and continue as normal. No backup protection is needed.
If you’ve missed two or more pills in a row (48 hours or more since you should have taken one), take the most recently missed pill right away and discard any others you skipped. Continue your pack on schedule, but use condoms or avoid intercourse for the next seven days. If those missed pills fell in the last week of active pills in your pack, skip the placebo (hormone-free) pills entirely, finish your active pills, and start a new pack the next day. This prevents a gap in hormone levels long enough for ovulation to occur.
Medications That Can Interfere
Several common medications speed up how your liver processes the hormones in birth control pills, potentially making them less effective. The most notable culprits include certain seizure medications (like carbamazepine, phenytoin, and topiramate), the antibiotic rifampin (used for tuberculosis), the antifungal griseofulvin, and some HIV medications. St. John’s Wort, an herbal supplement people take for mood, also reduces pill effectiveness. If you’re prescribed any new medication, it’s worth confirming whether it interacts with your contraceptive.
Over-the-Counter Access
For decades, all oral contraceptives in the U.S. required a prescription. That changed with the approval of Opill, a progestin-only pill now available without a prescription at pharmacies, big-box stores like Target and Walmart, and online retailers including Amazon. A one-month supply costs about $20, a three-month supply runs $50, and a six-month supply is $90. This makes it the first daily birth control pill available to anyone over the counter in the United States.
Combined pills still require a prescription because they carry more medical considerations, particularly around cardiovascular risk. But for people who don’t have contraindications to progestin-only pills, Opill removes the barrier of a doctor’s visit and provides a walk-in, no-questions-asked option at most major retailers.