How Do Birth Control Pills Work?

By Jennifer Nadel, MD
Medically reviewed checkmarkMedically reviewed
March 11, 2022

About 41% of women, or those who identify as women, between the ages of 15–24 are taking a birth control pill, because they have been proven to be a reliable form of birth control.

Before getting started with one, it’s good to know how they work in your body and what you should expect.

Some other names for birth control pills are oral contraceptives or simply just “the pill.” It can be rather daunting when looking at how many birth control pills are on the market. How do you choose the right one? 

In this article, we will discuss what birth control pills are, how effective they are, their side effects, and what happens if you miss a dose.

Finally, we will also review how much they cost and where you can get them. 

What Are Birth Control Pills

Birth control pills are little pills containing hormones that you usually take once a day.

Some have both estrogen and progesterone hormones, while others only have progesterone.

Both of these hormones are what help to prevent pregnancy.

The pills come in little trays with instructions on how to take your pill each day, and what to do if you miss a dose.

To work best, the pill should be taken at the same time every day.

Types of birth control pills

There are two types of birth control pills:

Combination pills: These pills have synthetic (i.e., lab-made) forms of the two hormones estrogen and progesterone. These hormones are naturally made in your body by your ovaries, and their levels fluctuate throughout your menstrual cycle. Changing the level of the hormones in your body is what prevents pregnancy.

When you get your birth control pills, it’s important to read over the directions and to make sure to ask your doctor or pharmacist about anything you don’t understand. The directions will review the schedule of how to take the pills. During the first few weeks, you will take active pills that have hormones in them.

Then, you may have a week of taking inactive pills, which have no hormones in them. This is the week that you will have your period. Even though your pills don’t have hormones in them, for that week you are still protected from getting pregnant.

Progesterone-only pills: These pills only have progestin (this is the name of synthetic progesterone). This pill is also called the mini pill. People who cannot take estrogen for health reasons can consider taking this birth control. With this method, all the pills in your tray will be active, and there are no inactive pills. You may or may not have your period while you take these pills.

People who have a history of stroke, heart disease, blood clots, are over 35 years of age, or are a smoker should consider taking this pill as opposed to the combination pill. This pill is also a good option for someone who is nursing a baby.

How effective is the pill

The pill has proven to be a reliable form of birth control.

According to the FDA, if 100 women (or those who identify as women) take their oral contraceptive pill according to the directions for one year, only nine of them will get pregnant.

It is crucial, however, that the pill be taken at the same time each day.

Pick a time of day that you know you will be home, and set a timer on your phone to remind you.

Taking the pill with some food or a glass of milk can help prevent nausea.

Side effects of taking birth control pills 

You may experience a few side effects when taking birth control pills:

Some rare but also more serious side effects include:

If you are a smoker, your risk for serious side effects is greatly increased. You should not smoke while taking birth control pills.

Have questions about birth control pills? Ask a doctor through K Health.

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How Do Birth Control Pills Work

The combination pills prevent ovulation. T

his means they prevent the egg from being released from your ovaries. They also thicken the mucus around your cervix and thin the lining of your uterus (womb). All these together help prevent pregnancy.

The mini pill slows the egg’s travel down your fallopian tubes.

It also thickens your cervical mucus and thins the lining of your uterus. Sometimes, the mini pill prevents an egg from being released from the fallopian tubes. 

Pregnancy prevention 

In order for there to be a viable pregnancy, sperm must come in contact with an egg that has been released from an ovary, and then the fertilized egg must implant itself on the wall of the uterus to grow.

Taking a combination pill helps stop the ovaries from releasing an egg each month.

Birth control pills also help thicken the mucus in front of your cervix, so that sperm have a difficult time getting past. Lastly, when the inner lining of the uterus is thin it makes implantation very difficult. 

STI protection

Birth control pills will not provide you with protection from sexually transmitted infections (STIs).

In order to be protected from STIs, including human immunodeficiency virus (HIV), you need to use a barrier like a condom each time you have sex.

How soon do birth control pills work

After you start taking your birth control pills, you need to use a backup method as well to prevent pregnancy for the first seven days.

After seven days, the medication is at the level in your body where it is working effectively and you are protected from getting pregnant.


When not taking the pill, your body ovulates once a month.

Ovulation is when an ovary releases an egg.

Each month one of your ovaries releases one egg, which then travels down your fallopian tubes and into your uterus. If the egg is not fertilized by a sperm, the body then expels it through your regular monthly cycle.

Preventing ovulation is the main way the combination pill prevents pregnancy. If no egg is released, then pregnancy cannot occur.

If an egg is released, the pill can make the fallopian tubes less likely to move the egg down into the uterus.

Cervical mucus

Under normal circumstances, the mucus around your cervix, the opening to your uterus, gets thinner when ovulation occurs.

When the mucus is thinner, it makes it much easier for sperm to get into your uterus where it may encounter an egg.

The second way the pill prevents pregnancy is by making the mucus around your cervix much thicker.

When the mucus in front of your cervix is thicker and stickier, sperm have a difficult time being able to pass through the cervix and to enter your uterus.


The endometrium is the lining of your uterus. Normally during ovulation, the endometrium becomes thick and ready for a fertilized egg to implant itself and grow there.

The third and final way the pill works is by preventing the endometrium from getting thicker. Instead, the lining thins, making it difficult for a fertilized egg to implant.

If a fertilized egg cannot implant, then it cannot continue to grow into a pregnancy.

Placebo week

Some combination birth control packets have what is called a placebo week.

This is a week when you take the placebo pills, which are “sugar pills”, that have no hormones in them.

It is still important to take the pills because it keeps you in the habit of taking your pill, and will prevent you from getting off schedule.

This is the week that you will have your period. Although you don’t have any hormones in your pills, you are still protected from getting pregnant.

The placebo schedule varies between brands, so be sure to read all the instructions enclosed with your pills, and ask your doctor or pharmacist about anything you don’t understand.  

Missing a pill 

If you happen to miss a pill, take that pill as soon as you remember, then resume taking your next pill at the time you usually take it. (Note, this might mean you take two pills on the same day.)

You should use a second method of birth control, like a condom, for the next 7–9 days to be sure that you don’t get pregnant.

How Can I Get Birth Control Pills

Birth control pills require a prescription from your doctor.

Schedule a visit with your doctor to discuss which type of birth control pills would be best for you.

Be sure to tell your doctor about any other medications or supplements you are taking to ensure they don’t interact with your birth control.

Telehealth doctors are also available. They can order you a birth control pill that can be delivered to your home.


The cost of birth control generally falls between $0–$50 a month. With most insurance plans, the pills can be totally free. 

Have questions about birth control pills? Ask a doctor through K Health.

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How long do birth control pills take to kick in?
After you start taking birth control pills, it takes seven days until you are protected from pregnancy. Be sure to use a second method of birth control (like a condom) during your first seven days of taking your birth control pills.
How can you tell if birth control is working?
After you start taking birth control, you might not have any noticeable signs that they are working. If you are following the instructions and taking them as directed, there is a 99% chance that they are working. If you are taking birth control pills for other reasons, such as hormonal acne or irregular bleeding, it can take up to four months to see a change.

K Health articles are all written and reviewed by MDs, PhDs, NPs, or PharmDs and are for informational purposes only. This information does not constitute and should not be relied on for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.

Jennifer Nadel, MD

Dr. Jennifer Nadel is a board certified emergency medicine physician and received her medical degree from the George Washington University School of Medicine. She has worked in varied practice environments, including academic urban level-one trauma centers, community hospital emergency departments, skilled nursing facilities, telemedicine, EMS medical control, and flight medicine.