How Does Plan B Work Before Sex?

Whether your condom breaks or you forget to take your birth control pill, Plan B (also known as the morning-after pill) can help prevent pregnancy. But how does it work?

Plan B works by inhibiting or delaying ovulation. It also thickens cervical mucus, making it harder for sperm to reach an egg and cause pregnancy.

How it works

The over-the-counter pill Plan B, also called the morning-after pill, works quickly to prevent pregnancy after unprotected sex or birth control failure. It contains the hormone levonorgestrel, a form of progesterone that can stop sperm from fertilizing an egg. Plan B works best if it is taken within three days (72 hours) of unprotected sex.

It can be effective up to five days after unprotected sex, but the chance of becoming pregnant decreases with each day that passes. The medication stops working when ovulation starts, so it’s important to take it as soon as possible after unprotected sex or contraceptive failure.

Some women have their periods early or late after taking the pill, but it’s totally normal. The pill disrupts the hormonal cyclical flow that causes your period, and it may delay your menstrual start or cause a lighter, heavier, or more spotty flow than usual.

There’s no harm in having sex after taking the morning-after pill, but it’s a good idea to use a condom or diaphragm for protection. It can take seven days for your ovaries to start producing hormones again, so if you have unprotected sex before that time, you might want to consider using another method of birth control to protect against a possible pregnancy. (You can find condoms and diaphragms in the family planning aisle, with no ID or prescription needed to buy them.)

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How to take it

Plan B is available over-the-counter for women ages 18 and older. It works best if you take it within the first three days of unprotected sex; after that, it may not be as effective.

The one-dose pill contains levonorgestrel, a progestin that prevents pregnancy by blocking the ovaries and uterus from releasing an egg. It can also help to delay ovulation. It can be used as emergency contraception if your birth control method fails (like if a condom breaks during sex or a birth control patch falls off) or you’re worried about unprotected sex. It can also be taken if your period is late or you missed your period due to illness or travel.

You can expect side effects from Plan B but they’re usually mild and pass quickly. A heavier next period is the most common side effect, but you might also experience spotting or lower stomach pain. If your bleeding is heavy or you’re passing large clots, contact your healthcare provider. These symptoms could be a sign of ectopic pregnancy, which happens outside the uterus and is a medical emergency.

You can also try another form of emergency contraception, such as ulipristal acetate (Ella). It’s similar to Plan B and works the same way but has a higher chance of being 95% effective in 24 hours and 89% effective in 72 hours.

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Side effects

A small number of people who take Plan B may experience side effects such as nausea, diarrhea, abdominal pain, or a change in vaginal discharge. These symptoms usually disappear within a few hours, but it is best to contact a doctor if they last longer or are severe. If vomiting or diarrhea persist, drink plenty of fluids, especially black, chamomile, or guava leaf tea. These drinks are easy to find in most grocery stores, and they can be purchased without a prescription.

It is important to note that Plan B only prevents pregnancy before ovulation, and the effectiveness of the medication decreases as time passes after taking it. If a person waits more than 72 hours to take it after unprotected sex, they will not be protected against pregnancy.

The active ingredient in Plan B that prevents pregnancy is levonorgestrel, which works by preventing ovulation and fertilization. It does this by delaying or stopping the release of an egg from the ovary, and it also makes it harder for sperm and an egg to meet.

A recent study found that a type of morning-after pill called ulipristal (ella(r)) is more effective than other forms of emergency birth control when taken as soon as possible after unprotected sex. Ulipristal can be a prescription-only option, and it must be prescribed by a doctor.


Plan B One-Step is 87-89% effective at preventing unplanned pregnancy when taken within 72 hours of an abortion, failure of regular birth control, or unprotected sex. It contains a larger dose of the progestin levonorgestrel found in birth control pills and works by interrupting the ovulation process. It also prevents sperm from fertilizing an egg and may alter the endometrium so that a fertilized egg cannot implant in the uterus.

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Unlike regular birth control, Plan B does not protect against sexually transmitted diseases (STDs). And while it usually works, it’s not the best option in terms of preventing pregnancy over the long term. It’s a good idea to start using a long-term form of birth control as soon as you can (like a pill, IUD or implant) to ensure the best results.

It’s important to remember that Plan B only lasts for five days, and the effectiveness decreases over time after taking it. This is why it’s best to keep a supply on hand at all times, especially if you forget your birth control or have unprotected sex. Despite its name, you don’t have to wait until morning to take it—it’s effective as soon as you can swallow it. Other forms of emergency contraception, like ella or ulipristal, have an even longer window of efficacy. However, ella requires a prescription and can be pricier than Plan B.

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