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Should You Take Plan B: Side Effects, What to Expect

  • The Pill Club
    Reviewed By: Shannon DeVita DNP, FNP-BC, CPNP-PC, Julie Lamonoff, CNM, OBGYN-NP

If you're reading this, you may feel like you need a plan B to prevent an unwanted pregnancy. Whether your birth control failed or you didn't use a birth control method, the Plan B One-Step® emergency contraceptive pill, AKA the morning-after pill, can be an effective way to prevent an unwanted pregnancy in the few days following unprotected sex.

But you also may be concerned about what side effects or disadvantages are involved in taking this emergency contraception pill.

We've previously covered how effective Plan B is and how taking Plan B can affect your menstrual cycle. In this article, we'll talk about potential side effects and what else you can expect from the Plan B pill.

What are the main Plan B emergency contraception side effects?

Side effects are an unfortunate part of taking medication, and Plan B side effects are no different. Some things you may experience include:

  • Nausea and/or vomiting

  • Lower abdominal pain

  • Fatigue

  • Dizziness

  • Headaches

  • Breast tenderness

  • Changes in menstrual bleeding

How long do you have to take Plan B?

Plan B One-Step is the most common brand name for the levonorgestrel 1.5mg tablet. It is an emergency contraceptive pill taken by mouth once as soon as possible after unprotected sex—the sooner you take it, the more effective it will be (preferably within 12 hours of having unprotected sex).

However, Plan B can prevent pregnancy if taken within 72 hours. If you take it within 24 hours of unprotected sex, it'll be 95% effective.

Plan B is easy to purchase online or at drugstores without a prescription. Planned Parenthood family planning clinics also offer access. Additionally, if you're a member of an online birth control telemedicine service like ours, you can get Plan B along with a birth control pill prescription.

Keep in mind that Plan B is not an abortion pill, and if you are already pregnant, it will not stop your existing pregnancy.

What is Plan B One-Step and how does it work?

What exactly happens to your body when you take Plan B? The active ingredient in the pill is called levonorgestrel. It's similar to the hormone progesterone that occurs naturally in your body.

This is a progestin hormone that works by preventing ovulation.

Ovulation is when an egg is released from a female's ovary and can be fertilized by male sperm for a potential pregnancy. But when you take Plan B, ovulation can be prevented, and an egg won't be released. Even if sperm are present, they won't be able to fertilize an egg, stopping pregnancy from occurring.

Plan B also may work by thickening the fluid on your cervix, which separates your vagina from your uterus. This makes it difficult for sperm to move into the correct position to produce a fertilized egg and a pregnancy.

Note: Plan B is 1 of 2 types of FDA-approved emergency contraceptive pills. The other form of emergency contraception is called ella™ (ulipristal acetate), but it's only available by prescription. And though it's not a pill, you can also use the Paragard® copper IUD as a form of emergency contraception. These are additional effective forms of emergency contraception, but both do require an evaluation and prescription from a healthcare professional.

Plan B: Everything you need to know about side effects

The active ingredient in Plan B, levonorgestrel, is the same ingredient used in many birth control pills, but at a higher dose. This means many potential side effects are similar to those of birth control pills that contain any type of progestin hormone, also known as combined oral contraceptives.

Most of these side effects, if you have any, will begin within a couple hours to a day of taking Plan B and will only last about two or three days.

Common side effects (occurring in >10% of women in clinical trials):

  • Heavier menstrual bleeding: This is the most commonly experienced side effect and most likely due to levonorgestrel's effect on your uterine lining. The shedding of this uterine lining is what causes normal bleeding during your menstrual period. The FDA reports occurrence in up to 31% of women.

  • Nausea/lower abdominal pain: Gastrointestinal issues like nausea or abdominal pain are common with hormonal change, and taking a high dose of levonorgestrel certainly counts as a hormonal change. The FDA reports these side effects in about 13% of women. If you do vomit within two hours of taking Plan B, you will most likely need to retake the pill, but you can call your healthcare provider or pharmacy to check if you are unsure.

  • Fatigue: Levonorgestrel has similar effects as the human hormone progesterone. When this hormone is at high levels, it can make you feel less energetic than you may be normally. Fatigue affects about 13% of women that take Plan B.

  • Headache: Hormones that regulate our menstrual cycle and pregnancy can also affect headache chemicals in the brain. About 10% of women experience headaches after taking Plan B.

Rare side effects (occurring in <4% of women in clinical trials):

  • Dizziness: About 10% of women report some dizziness while on Plan B.

  • Breast tenderness: Another effect of multi-tasking hormones is they can cause your breasts to swell a bit, potentially causing tenderness. About 8% of women have this.

  • Delayed period: You might experience menstrual changes, which can come early or late due to cycle interruptions from taking a high dose of levonorgestrel. We've covered this in more detail here. If your period is more than 5 days late, it's a good idea to take a pregnancy test or contact your healthcare professional. Most of the time your next period will be normal after taking Plan B.

Some women are concerned about the long-term effects of Plan B on fertility, or their ability to eventually get pregnant if they want to. It can be tough to accurately measure the long-term effects of Plan B use on fertility, because many different factors affect fertility, such as age and the health of your eggs.

That said, it's important to keep in mind that Plan B is meant for emergency contraception and should not be used as your regular form of birth control. If you are having sex regularly, there are many birth control options available to prevent pregnancy. Read about just a few options here and here.

If you're breastfeeding, you can take Plan B, but small amounts of progestins do pass into breast milk, shown as detectable levels in infants. However, this has not been found to impact the health, growth or development of the infant.

How to take Plan B

As soon as possible after you have unprotected intercourse (ideally within 24 hours), swallow the one pill that comes in the Plan B package. The cost of the brand name Plan B One Step pill is around $50. Other brands are available for much less, around $15 to $30.

If you take Plan B within 72 hours after unprotected sex, your chance of becoming pregnant is low, but not impossible. The only way to know for sure whether you're pregnant is to take a pregnancy test.

If your next period does not come within a week of when you expect it, it's a good idea to take a pregnancy test and seek medical advice from your OB/GYN or healthcare provider.

Keep in mind that no pills, IUDs, rings, implants or shots used for birth control also protect against sexually transmitted infections (STIs). Using a condom is always a good idea unless you and your partner only have sex with each other and have both been checked for STIs.


The most common side effects of the Plan B pill are relatively mild and do not last very long. Plan B is a very effective option for occasional instances where your regular birth control method fails. Your chance of avoiding an unwanted pregnancy is high as long as you take Plan B within 72 hours of unprotected sex, or even better, within 24 hours.

While Plan B has the potential to cause some unpleasant side effects, some women find that its benefits in preventing an unwanted pregnancy outweigh the risks of experiencing side effects.


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