Birth Control Pills: How They Work, Benefits & Side Effects

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The birth control pill (BCP) is a pregnancy prevention method that influences the production of estrogen and progesterone hormones. These hormones are responsible for pregnancy and various events in the menstrual cycle.

The BCP is a reversible hormonal contraceptive method that women of any age can take. It allows conception once women stop taking the medications. In the US, BCP is the most common form of contraception prescribed by doctors. The other hormonal contraceptives available are the patch, vaginal ring, and injection (1) (2).

Read this post to understand how birth control pills work, their usage, and their side effects.

What Is The Birth Control Pill?

Birth control pills (BCPs), also known as oral contraceptive pills (OCPs) or simply “the pill,” are medicines that contain synthetic versions of estrogen and progesterone (synthetic progesterone analogs are also termed as progestins). These pills are of three types:

  • Combined estrogen-progestin
  • Progestin-only
  • Continuous or extended use pills

The most common among them is the combined estrogen-progestin pill or the combination pill (1).

How Do Birth Control Pills Work?

In each menstrual cycle, one of the ovaries releases one mature egg by the process of ovulation. Birth control pills work by preventing this process. They may also exert their action by:

  • Thickening cervical mucus to restrict the entry of sperm into the uterus
  • Thinning the uterine lining to reduce chances of implantation (attachment of a fertilized egg to the uterus wall)

The typically prescribed birth control pills are combination pills and progestin-only pills, also known as “mini pills.” Oral contraceptives are available in 21- to 90-day pack, to be taken orally. The following sequence of active and inactive pills (hormone-free) is usually present.

  • 21 active pills and no inactive pills. Next set of active pills are started after a seven days gap. Menstruation occurs in this seven-day interval.
  • 21 active pills followed by seven inactive or placebo pills (usual menstrual bleeding occurs during these last seven days). The function of the inactive pills is to maintain the habit of taking a pill daily.
  • 28 active pills (no breaks, as seen with progestin-only pills).
  • In extended cycle dosing, inactive pills are present only up to four times a year. So you have your periods only four times a year when you consume the inactive pills.
  • The extended cycle pills have continuous active pills for three months, followed by seven days of inactive ones. So you have your periods once every three months (1)( 3).

These oral contraceptives do not reduce the chances of contracting sexually transmitted diseases (STDs) such as AIDS, syphilis, and gonorrhea, and you will have to use a condom to prevent STDs ( 3).

How Soon Do Birth Control Pills Work?

Pregnancy should be ruled out before starting any regimen with OCPs. If you start the pill on day one of your period, then no extra measures are needed. Otherwise, it takes about seven days for the pill to be functional. Until then, an alternative contraceptive method, such as condoms, should be used during sexual intercourse (4).

What To Do If A Pill Is Missed?

The FDA reports a nine in a 100 chance of getting pregnant with the typical use of this method. This is seen to further reduce with strict adherence. Therefore, it is very important that you take a pill daily, around the same time of the day, whether or not you have sexual activity.

The inactive pills or pill-free days may be decided according to the type of pill prescribed and the instructions on the package. If a day’s pill is missed or the start of a new pack is delayed, you should:

  • Take it as soon as you remember it, even if it means taking two pills the next day and then resume one pill per day
  • Use additional protection, such as condoms and a spermicide, for the next five to seven days (5).
  • If you miss three pills in a row, do not take the missed pills together. Instead, use other contraception methods and contact your healthcare provider for advice.

What Are The Benefits Of Birth Control Pills?

The birth control pills may offer the following advantages (6) (7).

  • No interruption in sexual activity
  • Fully reversible method
  • May make periods regular and less painful
  • May help with polycystic ovary syndrome (PCOS)
  • May reduce premenstrual syndrome (PMS)
  • May reduce risk of ovarian cancer, endometrial cancer, and colorectal cancer
  • May sometimes be used to reduce acne
  • May reduce the occurrence of fibrocystic breast disorders.

What Are The Side Effects Of Birth Control Pills?

Birth control pills generally show mild side effects, which disappear with continued usage. Switching to a different pill formulation after doctor consultation may also solve some of the problems.  Some side effects reported with BCPs are (1)( 3):

  • Break through vaginal bleeding
  • Painful periods with intermittent spotting
  • Nausea
  • Vomiting
  • Diarrhea
  • Weight gain or loss
  • Decreased or increased appetite
  • Gingivitis (swelling in gums)
  • Abdominal cramps
  • Hyperpigmentation
  • Hair growth at unusual places
  • Tenderness in breasts or any discharge from them
  • Increased vaginal discharge or inflammation
  • Loss of libido (sex drive)

Besides these effects, irregular intake of birth control pills may make periods irregular. Therefore, you must take the pills as prescribed and on time. Consult your doctor if any of these side effects persist or become severe.

Are There Health Risks Associated With Birth Control Pills?

Birth control pills are not recommended for women who are older than 35 years and are smokers (even if they recently quit smoking). BCPs are also contraindicated in women with a history of:

  • Heart attack
  • Stroke
  • Clotting disorders
  • Liver disorder
  • Breast cancer

Women with underlying disorders such as chronic diabetes, hypertension, high cholesterol, or other cardiovascular disorders should always consult their physician before starting BCPs. A majority of the risks are associated with the estrogen in the BCPs, and replacing a combination pill with a progestin-only pill may be suggested by your doctor (8).

Is It Ok To Take Other Drugs While Taking BCPs?

Certain medications and even herbal formulations are capable of interacting with BCPs and reduce their effectiveness. Hence, any ongoing medication should be communicated to the healthcare provider to rule out interactions (9).

Drugs that reduce BCP action are:

  • Antibiotics (rifampicin, rifabutin)
  • Antivirals (ritonavir, darunavir, nevirapine)
  • Anti-seizure drugs (carbamazepine, phenytoin, lamotrigine, felbamate)
  • Antifungals (griseofulvin)

Among herbal formulations, those containing St. John’s Wort have been reported to cause the failure of contraception with BCPs (10).

Is The Pill Safe For Breastfeeding Mothers?

The estrogen content of the combination pills may lead to decreased milk production. Therefore, your doctor may advise resuming pills after six weeks of birth or switching to mini pills or progestin-only pills (3) (11). Progestin-only pills do not affect milk supply or your baby’s health.

How Much Do The Pills Cost?

The BCPs are available at an average cost of $5 to 39 per month. They may also be available free of cost at some government healthcare centers in some countries (6) (8).

How Are The Birth Control Pills Packaged?

Generally, active (with hormones) and inactive (hormone-free) pills are colored differently. The package could contain pills in any of the following variations.

  • 21 active pills
  • 21 active pills and 7 inactive pills, where inactive/dummy pills may contain dietary supplements, such as iron
  • 90 days pill pack with a pill daily for 84 days; the last 7 pills are either hormone-free or estrogen-only
  • 365 days pack with a pill for each day

What Are The Alternatives To Birth Control Pills?

There are several alternatives to pills when it comes to hormonal methods of contraception (12).

  • Implant: Small progestin-containing rod inserted under the skin of upper arm; it can release progestin up to three years
  • Injection: Once-in-three-months shot of progestin
  • Combined contraceptive patch: Once-a-week patch containing a combination of estrogen and progestin to be worn on the skin (lower abdomen, buttocks, upper body except on breasts) continuously for three weeks and patch-free fourth week
  • Combined contraceptive vaginal ring: When placed in the vagina, releases combined estrogen and progestin hormones (to be taken out for a week during your period)

There are non-hormonal contraceptive methods as well, such as male and female condoms, spermicide, cervical cap, contraceptive sponge, intrauterine devices, and surgical sterilization (13).

Frequently Asked Questions

1. Does the pill cause weight gain?

There is no direct link between weight gain and the usage of birth control pills. However, side effects of BCPs include alteration in appetite and gastric disturbances. So, initially, it may contribute to water weight and bloating, leading to a feeling of heaviness (14)(15).

2. Does the pill cause increased breast size?

It may cause a reversible increase in breast size since breast tissue growth depends on the same hormones present in the pills. The effects generally wear off with consecutive cycles.

3. Does the pill help clear acne?

Yes, doctors may prescribe combined oral contraceptive pills and other medications to treat acne in some women (16).

4. Does alcohol intake interfere with the pill?

No, alcohol does not alter the effectiveness of birth control pills. In case you vomit after drinking within two hours of taking your pill, it may come out of your body. However, BCPs slow down the rate at which alcohol is eliminated from the body. So, women on BCPs are likely to feel the effects of alcohol for a longer duration (17).

The birth control pill is an effective method to prevent pregnancy. As per WHO, its effectiveness can reach up to 99.7% with continuous and correct use (12). It can also help you manage many of your period-related problems. However, do share your full medical history with your healthcare provider to get what best suits your requirements.

Key Pointers

  • The three forms of oral contraceptive pills are combined estrogen-progestin, progestin-only, and continuous or extended use pills.
  • Oral contraceptives come in packets of 21 to 90 days.
  • Before initiating any oral contraceptive pills, be sure you are not pregnant.
  • OCPs should not be taken by women over 35, smokers, or those with a history of heart attack, cancer, stroke, liver, or clotting disorders.


MomJunction's articles are written after analyzing the research works of expert authors and institutions. Our references consist of resources established by authorities in their respective fields. You can learn more about the authenticity of the information we present in our editorial policy.
  1. Danielle B Cooper and HebaMahdy; (2021); Oral Contraceptive Pills.
  2. Combined Hormonal Contraceptives.
  3. Estrogen and Progestin (Oral Contraceptives).
  4. The Pill.
  5. Birth Control.
  6. Combined Pill.
  7. Oral Contraceptives and Cancer Risk.
  8. Birth Control Pills.
  9. Haiying Sun et al.; (2020); Drug-Drug Interaction Studies With Oral Contraceptives: Pharmacokinetic/Pharmacodynamic and Study Design Considerations.
  10. Simon Nicolussi et al.; (2020); Clinical relevance of St. John\’s wort drug interactions revisited.
  11. The progestogen-only pill.
  12. Family planning/contraception methods.
  13. Contraception.
  14. Combined Hormonal Birth Control: Pill
  15. Contraception: Do hormonal contraceptives cause weight gain?
  16. Trivedi et al., M. K. “A Review of hormone-based therapies to treat adult acne vulgaris in women.” 2017,
  17. Alcohol.
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Dr. Joyani Das

Dr. Joyani Das did her post-graduation from Birla Institute of Technology, Mesra and PhD in Pharmacology. Previously, she worked as an associate professor, faculty of Pharmacology, for two years. With her research background in preclinical studies and a zeal for scientific writing, she joined MomJunction as a health writer. Her research work was published in international journals and publications, such... more

Dr. Indira Vijayakrishnan

Dr. Indira Vijayakrishnan is a consultant Ob/Gyn at Amar Maternity and Fertility Centre in Attingal, Kerala. She is in charge of the Amar Centre for Advanced Reproduction. An expert in obstetric sonology, Dr. Vijayakrishnan is also trained in fetal echocardiography and deals with high-risk pregnancies and antenatal care. In addition to being a member of the Royal College of Obstetricians... more