Oral contraceptives are prescription pills containing synthetic hormones that prevent pregnancy. They’re one of the most widely used forms of birth control, with a typical use failure rate of about 7% per year. The pills work primarily by stopping ovulation and thickening cervical mucus so sperm can’t reach an egg.
How Oral Contraceptives Work
The hormones in oral contraceptives prevent pregnancy through two main mechanisms. First, they suppress ovulation, meaning your ovaries don’t release an egg each month. Second, they thicken the mucus at the opening of the cervix, creating a barrier that makes it much harder for sperm to get through. Without an egg to fertilize and with reduced sperm penetration, the chances of pregnancy drop dramatically.
Combined Pills vs. Progestin-Only Pills
There are two main types of oral contraceptives, and the difference comes down to hormones. Combined oral contraceptives (often just called “the pill”) contain two synthetic hormones: a form of estrogen and a form of progestin. The progestin-only pill, sometimes called the mini-pill, contains just one hormone. Both types have the same typical failure rate of 7% per year, according to the CDC, though that number reflects real-world use where people occasionally miss pills or take them late. With perfect use, the failure rate is considerably lower.
The progestin-only pill is often a better fit for people who can’t take estrogen, such as those with a history of migraines with aura, certain heart conditions, or those who are breastfeeding.
Pill Schedules and Formats
Combined pills come in several formats depending on how often you want a period. Cyclic formulations are the most common: you take active hormone pills for 21 to 24 days, followed by 4 to 7 days of inactive (placebo) pills, during which you’ll typically have withdrawal bleeding. Extended cycle formulations stretch the active pills to three months, so you only bleed four times a year. Continuous use skips the placebo pills entirely for a full year, effectively stopping menstrual bleeding altogether.
Within these schedules, pills can be monophasic (every active pill has the same hormone dose) or multiphasic (the dose changes week to week throughout the pack). Your prescriber chooses based on your health profile and how your body responds.
Side Effects
Most people who take oral contraceptives notice some side effects, especially in the first few months. In one large survey, over 80% of people reported side effects during their first year of use, though that number dropped to around 53% among those who had been taking them for more than 10 years. Breast tenderness is one of the more common complaints, affecting roughly 37% of combined pill users and 28% of those on progestin-only pills. Bloating, increased appetite, a feeling of heaviness, and changes in vaginal dryness each show up in about 25% to 30% of users.
Breakthrough bleeding or spotting between periods is also common, particularly in the first few cycles. These side effects generally improve as your body adjusts to the hormones.
Serious Risks to Know About
The most significant risk associated with combined oral contraceptives is blood clots, specifically a type called venous thromboembolism (VTE). For women of reproductive age not using hormones, the baseline risk is about 1 to 5 clots per 10,000 women per year. Combined pill use raises that to roughly 3 to 15 per 10,000 women per year. For context, that risk is still lower than the clotting risk during pregnancy itself, and it tends to decrease the longer you take the pill.
Certain factors push that risk higher. Smoking is a major one, particularly if you’re over 35 and smoke 15 or more cigarettes a day. That combination is considered an unacceptable health risk for combined pills. Obesity, high blood pressure, and inherited clotting disorders also increase the danger. The type of progestin matters too: pills containing newer progestins like drospirenone carry a slightly higher clotting risk (10 to 15 per 10,000 women per year) compared to older formulations using levonorgestrel (about 8 per 10,000).
Who Should Not Take Combined Pills
Based on the CDC’s 2024 medical eligibility criteria, several conditions make combined hormonal contraceptives an unacceptable risk:
- Smokers 35 or older who smoke 15 or more cigarettes daily
- History of blood clots or known clotting disorders
- Migraine with aura at any age
- Current breast cancer
- Severe high blood pressure (systolic 160 or above, or diastolic 100 or above)
- History of heart attack or stroke
- Certain liver tumors
- Complicated heart valve disease
People with these conditions may still be able to use the progestin-only pill, which carries fewer cardiovascular risks. The decision depends on the specific condition and should be made with a prescriber.
Health Benefits Beyond Pregnancy Prevention
Oral contraceptives are prescribed for a range of conditions that have nothing to do with preventing pregnancy. They’re effective at reducing painful periods and heavy menstrual bleeding. Pills containing certain progestins with anti-androgenic properties can treat acne, excess hair growth, and oily skin. For people with polycystic ovary syndrome (PCOS), oral contraceptives can normalize ovarian structure and manage symptoms.
There’s also strong evidence that long-term use reduces the risk of endometrial, ovarian, and colon cancers. These protective effects can persist for years after stopping the pill.
What to Do If You Miss a Pill
Missing pills is the most common reason oral contraceptives fail in real-world use. The CDC’s guidelines break it down by how long it’s been since you should have taken your pill.
If you’re less than 24 hours late, take the pill as soon as you remember and continue the pack as normal. No backup contraception is needed. If you’ve missed one pill by 24 to 48 hours, the same advice applies: take it as soon as possible, even if that means taking two pills in one day. You’re still protected.
Missing two or more consecutive pills (48 hours or longer) is where things change. Take the most recent missed pill right away and discard any others you skipped. Then use condoms or avoid intercourse for the next seven days while you continue taking pills. If those missed pills fell during the last week of active pills in your pack, skip the placebo week entirely and start a new pack immediately. If you missed pills during the first week and had unprotected sex in the previous five days, emergency contraception is worth considering.
Medications That Reduce Effectiveness
Certain drugs speed up how your liver processes estrogen, which can drop hormone levels low enough to cause contraceptive failure. The most well-documented culprits are anti-seizure medications that act as enzyme inducers, such as carbamazepine and phenytoin. These drugs ramp up the liver’s processing speed, leaving less active hormone circulating in your body. If you take any anti-seizure medication, it’s worth confirming whether it interacts with your pill. People on these medications often need a different contraceptive method altogether, such as an IUD, rather than simply adjusting the pill dose.