Does Plan B Have Side Effects? What Research Says

What Are the Most Common Side Effects of Plan B?

Research and clinical trials have established a clear list of common side effects. These occur because the sudden surge of hormones affects your body beyond just your ovaries.

The most frequently reported side effects include:

• Nausea — This is the most common complaint. Some women vomit, which is why some doctors suggest taking an anti-nausea medication beforehand if available.
• Fatigue — Feeling unusually tired for a day or two is normal.
• Headache — Hormonal shifts can trigger this.
• Breast tenderness — Similar to what you might feel before a period.
• Dizziness — This is less common but still reported.
• Lower abdominal pain — Cramping that feels like a period is common.

These symptoms typically begin within a few hours of taking the pill and usually resolve within 24 to 48 hours. They do not require medical treatment. If you vomit within two hours of taking the pill, the dose may not be fully absorbed, and you should contact a healthcare provider to discuss whether you need a second dose.

How Does Plan B Affect Your Menstrual Cycle?

The most significant and often most concerning side effect is a change to your next period. Because the medication works by delaying ovulation, it directly alters your cycle timing.

Your next period may arrive earlier than expected or later than expected. The timing depends on where you were in your cycle when you took the pill. If you took it just before ovulation, the delay may push your period back by several days.

The flow may also be different. Some women report a heavier period, while others report a lighter one. Spotting between periods is also possible.

The key point to remember is that your period should return to normal by the following cycle. If your period is more than seven days late, you should take a pregnancy test. A late period is not necessarily a sign of pregnancy, but it is the standard threshold for checking.

Why Does the Hormone Dose Cause These Reactions?

Plan B contains 1.5 mg of levonorgestrel. A typical daily birth control pill contains about 0.1 mg. That is a 15-fold difference in a single dose.

Your body is not used to receiving that much synthetic progesterone at once. The hormone travels through your bloodstream and binds to receptors in your brain, specifically the pituitary gland. This binding sends a signal to stop the surge of luteinizing hormone (LH) that normally triggers the ovary to release an egg.

The side effects are the result of this sudden hormonal spike. They are not a sign that the medication is working or failing. They are simply the body’s response to a large dose of a hormone it normally sees in much smaller amounts.

Does Plan B Have Side Effects That Are Serious?

Serious side effects are rare but they do exist. You should know the difference between common discomfort and a reason to seek medical care.

Severe abdominal pain, especially on one side, can be a sign of an ectopic pregnancy. This is a pregnancy that implants outside the uterus, usually in a fallopian tube. Plan B does not cause ectopic pregnancies, but if you become pregnant after taking it, you should be aware of the risk. If you experience sharp, one-sided pain, especially with dizziness or shoulder pain, seek emergency care.

Heavy bleeding is another concern. If you soak through more than one pad or tampon per hour for several hours, this is not normal menstrual bleeding. Contact a healthcare provider.

Some women experience severe allergic reactions, though this is extremely rare. Swelling of the face, lips, or tongue, or difficulty breathing, requires immediate emergency attention.

Does Plan B Cause Weight Gain or Long-Term Fertility Issues?

No clinical evidence supports the claim that emergency contraception causes weight gain. The medication is in your system for a short period, and it does not alter your metabolism or fat storage over the long term.

There is also no evidence that Plan B affects future fertility. Taking it once, or even multiple times in a single cycle, does not damage your reproductive system. It does not thin your uterine lining permanently, and it does not reduce your chances of becoming pregnant in future cycles.

The confusion often comes from the difference between emergency contraception and regular birth control. Daily birth control pills can have long-term effects on your cycle, but a single emergency dose is a one-time event. Your body clears the hormone within a few days, and your normal hormonal rhythm resumes.

Does Plan B Have Side Effects That Differ From Other Emergency Contraception?

There is another type of emergency contraception that works differently. It contains a medication called ulipristal acetate, sold under the brand name Ella. It is available by prescription only.

The side effect profiles are similar, but there are differences. Ulipristal acetate is less likely to cause nausea, but it is more likely to delay your period by a few extra days. It also requires a doctor’s visit to obtain.

The copper IUD is also used as emergency contraception. It is the most effective option, but it has a different set of side effects. Insertion can be painful, and it may cause heavier periods and cramping for the first few months.

Does Plan B Have Side Effects That Affect How Well It Works?

Your body weight can influence how effective Plan B is. Research has shown that levonorgestrel-based emergency contraception is less effective in women who weigh more than 165 pounds (75 kg). It is not completely ineffective, but the failure rate is higher.

This is not a side effect, but it is directly related to how the medication works in your body. If you weigh more than this threshold, you should talk to a healthcare provider about alternatives. Ella or a copper IUD may be more reliable options for you.

Vomiting is the other factor. If you vomit within two hours of taking the pill, the medication may not have been fully absorbed. You should contact a healthcare provider to discuss whether a repeat dose is appropriate. Do not take a second dose on your own without guidance.