Birth Control Pills: Types, How They Work, and Side Effects

More than 14 percent of women aged 15 to 49 in the United States currently use birth control pills, making oral contraceptives one of the most common contraceptive methods. The category includes two distinct medication types — combined pills and progestin-only pills — dozens of formulations, and a long list of secondary uses beyond pregnancy prevention. Choosing among them depends on personal health history, lifestyle, and which benefits or side effects matter most. This article is educational information, not medical advice; a prescriber (or, for the over-the-counter option, the package labeling) should guide your specific choice.

How Birth Control Pills Work

Combined oral contraceptives contain both estrogen (typically ethinyl estradiol) and a progestin. They work primarily by suppressing ovulation through inhibition of the hypothalamic-pituitary-ovarian axis, stopping the hormone surges needed to release an egg. They also thicken cervical mucus to block sperm and thin the uterine lining to make implantation less likely. ACOG notes combined pills are about 99 percent effective with perfect use and roughly 93 percent effective with typical use.

Progestin-only pills (POPs, often called the mini-pill) contain only a progestin. They primarily thicken cervical mucus and can suppress ovulation in some cycles. Traditional norethindrone-based POPs require strict timing (taking the pill within about 3 hours of the same time daily). A newer drospirenone-based POP (Slynd) has a more forgiving 24-hour window with a grace period. Opill (norgestrel), the over-the-counter option discussed below, is also a progestin-only pill. POPs are broadly similar to combined pills in effectiveness when used correctly, with typical-use effectiveness around 91 percent.

Types of Birth Control Pills

Combined pills come in monophasic (same hormone dose throughout the cycle), biphasic, triphasic, and four-phasic formulations. Many people do equally well on monophasic pills. Packs come in 21-day, 24-day, and 28-day formats. Extended-cycle and continuous-cycle regimens (such as Seasonale, Seasonique, and Amethyst) reduce period frequency to about four times per year or eliminate scheduled bleeding entirely. The choice often comes down to managing menstrual issues such as heavy bleeding, painful periods, PMS, or endometriosis.

Progestin-only pills are appropriate for many people who cannot use estrogen — including those with a history of blood clots, certain migraines, current or past breast cancer (discuss with your clinician), uncontrolled hypertension, or who are recently postpartum and breastfeeding. POPs have a narrower set of contraindications than combined pills.

Different progestins (levonorgestrel, norethindrone, drospirenone, desogestrel, norgestimate, norgestrel, and others) have varying side-effect profiles. Some are more androgenic (potentially worsening acne), while others have antiandrogenic activity and may improve skin. Drospirenone has mild diuretic activity, which can help with bloating but can raise potassium in some people, so clinicians use caution with certain kidney, liver, or adrenal conditions or with some other medications.

Effectiveness: Typical Use vs Perfect Use

Perfect-use effectiveness assumes the pill is taken correctly every day, on time. Typical-use effectiveness reflects real-world adherence. CDC contraception data show typical-use failure rates around 7 percent per year for the pill, primarily because people miss pills, take them late, or experience drug interactions.

Effectiveness can drop with concurrent use of certain medications, including some seizure medications (carbamazepine, phenytoin, topiramate at higher doses), the antibiotics rifampin and rifabutin, some HIV medications, and St. John’s Wort. Most other antibiotics do not meaningfully reduce pill effectiveness, despite a persistent myth. Vomiting or severe diarrhea within a few hours of taking a pill can count as a missed pill. Always tell your prescriber and pharmacist about every medication and supplement you take.

Who Should Not Use Combined (Estrogen) Pills: CDC US MEC Screening

Estrogen-containing pills are not safe for everyone, which is why a prescriber screens your medical history before prescribing them. The CDC U.S. Medical Eligibility Criteria for Contraceptive Use (US MEC, 2024 edition) classify each method for each condition into four categories:

• Category 1 — no restriction on use.
• Category 2 — advantages generally outweigh risks.
• Category 3 — risks usually outweigh advantages (use generally not recommended unless other methods aren’t available or acceptable).
• Category 4 — unacceptable health risk (the method should not be used).

For combined hormonal contraceptives, conditions that fall into Category 3 or 4 (meaning combined pills are generally not recommended or should be avoided) include, among others:

• Migraine with aura, at any age
• Smoking at age 35 or older (risk rises with the number of cigarettes)
• Uncontrolled or significantly elevated high blood pressure, and hypertension with vascular disease
• Current or past venous thromboembolism (blood clots — DVT/PE), or a known thrombogenic (clotting) mutation
• Less than 21 days postpartum (and longer if breastfeeding or if other clot risk factors are present)
• Current breast cancer
• History of ischemic heart disease or stroke
• Complicated valvular heart disease, or systemic lupus with positive/unknown antiphospholipid antibodies
• Severe (decompensated) liver disease, certain liver tumors, or diabetes with vascular complications
• Major surgery with prolonged immobilization

This is not a complete list, and it is not something to self-assess — the point is that a clinician (or, for the OTC pill, the labeling and your pharmacist) reviews your history to determine whether an estrogen-containing pill is appropriate. When combined pills are not a good fit, progestin-only pills, an IUD, an implant, or other methods are often safe alternatives.

Side Effects and Serious Risks

Common side effects of combined pills include nausea (often resolving within a few weeks), breast tenderness, mood changes, headaches, breakthrough bleeding (especially in the first 3 months), and changes in libido in some users. Most side effects improve after 2 to 3 cycles as the body adjusts.

Serious but uncommon risks include venous thromboembolism (roughly 3 to 9 cases per 10,000 woman-years on combined pills, versus about 1 to 5 per 10,000 among non-users who are not pregnant), stroke, and heart attack; with prolonged use there are also small changes in breast and cervical cancer risk. This absolute risk is low for most healthy users but is concentrated in the higher-risk groups the US MEC identifies — smokers over 35, people with hypertension, migraine with aura, or a personal or strong family history of clotting disorders. That is exactly why the screening above matters.

POPs carry fewer cardiovascular risks because they contain no estrogen. Their main side effect is irregular or unpredictable bleeding (very common in the first months), with some breast tenderness or mood changes. POPs are generally safer than combined pills for people with migraine with aura, hypertension, or a VTE history — though your clinician still reviews your full history.

Warning Signs to Act On: “ACHES”

While serious complications are rare, everyone taking combined hormonal contraception should know the classic warning signs, often remembered with the mnemonic ACHES. Seek prompt medical care if you develop:

• A — severe Abdominal pain
• C — Chest pain, shortness of breath, or coughing up blood
• H — severe Headaches, especially new or worsening, or with dizziness or weakness
• E — Eye problems such as vision loss or blurring, or trouble speaking
• S — Severe leg pain, swelling, or warmth (a possible clot)

These can signal a blood clot, stroke, or other serious problem. If you have symptoms that could be a clot in the lung, a stroke, or a heart attack, call 911 or go to the nearest emergency room.

Non-Contraceptive Benefits

Combined pills reduce ovarian and endometrial cancer risk by roughly 30 to 50 percent with prolonged use, and that protection can persist for years after stopping. They reduce the risk of benign breast disease, functional ovarian cysts, and ectopic pregnancy. Many people take combined pills for non-contraceptive reasons including acne, hirsutism, polycystic ovary syndrome, heavy menstrual bleeding, painful periods, premenstrual dysphoric disorder, and endometriosis-associated pain. Continuous and extended-cycle use can substantially improve menstrual-related conditions by reducing or eliminating scheduled bleeding, and skipping the placebo week is generally considered safe when done under a clinician’s guidance.

Who Birth Control Pills Are Right For

Combined pills suit people who can take a daily pill consistently, do not smoke (or are under 35 and don’t smoke), and do not have the contraindications listed above. They work well for those who want strong cycle control or relief from menstrual symptoms. Progestin-only pills suit people who are postpartum and breastfeeding, who are over 35 and smoke, who have certain migraine patterns, or who otherwise cannot use estrogen; the drospirenone-based POP offers more forgiving timing.

For those who want comparable or higher effectiveness without daily dosing, longer-acting reversible options like an IUD or implant offer typical-use effectiveness above 99 percent without daily action. The conversation about contraception is best framed around personal priorities rather than ranking methods, and switching methods over a lifetime is normal and expected.

Cost and Access

Most generic combined pills cost roughly $0 to $50 per month with insurance, and often $0 under Affordable Care Act preventive-services coverage. Without insurance, generic pills commonly run about $15 to $50 per month at retail; discount programs (GoodRx, Cost Plus Drugs) have pushed cash prices below $20 per month for many generics. Pharmacist-prescribed birth control is now available in many states, and telehealth services routinely prescribe oral contraceptives — see our telehealth guide — often at lower total cost than an in-person visit.

Opill (norgestrel 0.075 mg), a progestin-only pill, was approved by the FDA in July 2023 as the first daily oral contraceptive available in the U.S. without a prescription and reached store shelves in 2024. It costs roughly $20 per month for a one-month supply or about $89 for a six-month supply. Opill is not for emergency contraception and does not protect against sexually transmitted infections; per the FDA labeling, it should not be used by people with a history of breast cancer or alongside another hormonal contraceptive. The intersection of contraception and healthcare costs matters considerably for people paying out of pocket.

Frequently Asked Questions

How long does it take for birth control pills to work?

If a combined pill is started within 5 days of the first day of your period, it is generally effective right away. Started at another time, use a backup method for the first 7 days. POPs are typically considered effective after about 48 hours of consistent use. Follow the timing guidance in your pill’s package or from your clinician.

Will I gain weight on the pill?

Large studies have not shown a consistent weight-gain effect from combined birth control pills. Some people notice mild water retention in the first cycles. The contraceptive injection (depot medroxyprogesterone, “the shot”) is more associated with weight gain than oral pills.

Can I skip my period on the pill?

Yes, with combined pills. Skipping the placebo week and starting a new active pack extends the hormone phase without a scheduled bleed; continuous-cycle pills are designed for this. Breakthrough bleeding is common at first but usually settles. Most clinicians consider this safe for most people, but confirm it fits your situation.

What should I do if I miss a pill?

General guidance is to take a missed combined pill as soon as you remember and continue the pack; if you are more than about 48 hours late (or miss more than one), backup contraception for 7 days is usually advised, and emergency contraception may be worth considering if you had unprotected sex. POP timing rules are stricter, especially for older formulations. Because the exact steps depend on your specific pill and where you are in the pack, follow the instructions in your pill’s package insert or ask your pharmacist or clinician.

What to Take Away

Birth control pills offer a flexible, effective, and well-studied contraceptive option with meaningful non-contraceptive benefits. The right choice depends on health history, daily routine, and personal priorities. Combined pills give strong cycle control and broader benefits but carry estrogen-related risks, which is why a prescriber screens for the CDC US MEC contraindications before prescribing them. Progestin-only pills are safer for people with cardiovascular risk factors but require tighter timing for older formulations. With an OTC progestin-only option (Opill), app-based prescribing, and pharmacist-prescribed pills now widely available, access has improved substantially — and knowing the ACHES warning signs helps you use any of these options safely.

Sources

• Centers for Disease Control and Prevention — U.S. Medical Eligibility Criteria for Contraceptive Use (2024) and U.S. Selected Practice Recommendations
• American College of Obstetricians and Gynecologists (ACOG) — Combined Hormonal Birth Control
• U.S. Food & Drug Administration — FDA Approves First Nonprescription Daily Oral Contraceptive (Opill)
• MedlinePlus (U.S. National Library of Medicine) — Birth Control