Some people who take Plan B experience bleeding for up to 1 month afterward, but this typically goes away on its own. In some cases, Plan B may also cause irregular periods or bleeding between periods.

Plan B is an emergency contraception that may prevent pregnancy. It contains a synthetic form of a naturally occurring hormone, which allows it to prevent ovulation. It does not cause an abortion.

Keep reading to learn more about how Plan B works, some possible side effects, and what to do if this form of emergency contraceptive is ineffective.

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Heavy bleeding is not a common side effect of Plan B.

Most people who take Plan B do not experience bleeding, but it does happen. A 2006 study involving 232 participants found that 14.7% experienced bleeding between periods after using Plan B.

The Food and Drug Administration (FDA) report that heavier periods are relatively common after Plan B, occurring in as many as 31% of people who have taken the drug.

However, a person should not expect to experience very heavy bleeding after using Plan B — even days or weeks later.

Some people believe that heavy bleeding is a common side effect of Plan B. This is not true. The belief may stem from the misconception that Plan B causes an abortion. In fact, Plan B does not cause pregnancy loss or the bleeding that can accompany it.

People who frequently use Plan B may find that their periods become irregular. This is because the progestin can disrupt the natural menstrual cycle, and it can take about 1 month for the body to readjust.

If a person relies on emergency contraception regularly, they may benefit from switching to a form of birth control that does not involve taking daily pills, such as an intrauterine device, or IUD.

Plan B is the branded form of the generic drug levonorgestrel. This is one type of progestin, a synthetic form of the hormone progestogen, which the body naturally produces.

People commonly refer to Plan B as the “morning-after pill.”

When penetrative sex can lead to pregnancy, the chances are higher if sex occurs right before or right after ovulation, when an ovary releases an egg.

Like other birth control pills, Plan B works by either preventing the ovary from releasing an egg or by delaying the egg’s release.

If ovulation occurs after taking Plan B, the drug can help reduce the risk of the egg implanting in the uterus, leading to pregnancy.

Using Plan B involves taking a single pill right away and a second pill 12 hours later. The two doses are identical — both contain 0.75 milligrams of levonorgestrel.

Emerging research suggests that taking the two pills at the same time may be just as effective as taking them separately.

Plan B can work for up to 120 hours after unprotected sex, though prescribing guidelines generally recommend starting the dosage within 72 hours of unprotected sex. The earlier a person takes the drug, the better it may work.

Plan B does not cause an abortion or affect future fertility.

Also, while no research has specifically studied women who became pregnant after using Plan B, there is no evidence that taking any progestin-based birth control method increases the risk of abnormalities in fetal development.

Most research on Plan B is somewhat dated. Various studies have found effectiveness rates ranging from 52–100%.

Whether Plan B can successfully prevent pregnancy depends, in part, on how soon a person takes it and the stage of their menstrual cycle.

Plan B may be less effective in people who weigh more than 165 pounds. However, very little research supports this idea. Also, the drug poses no additional risks to people who weigh more than this, so it is likely still worth considering.

Side effects of Plan B are similar to those of other birth control pills. The FDA have reported that, according to results of trials, the most common side effects include:

  • nausea, in 14% of study participants
  • pain in the lower abdomen, in 13%
  • fatigue, in 13%
  • dizziness, in 10%
  • a headache, in 10%

Hormonal contraceptives are not safe for everyone. However, the effects of single-use emergency contraceptives, such as Plan B, are not the same as the effects of long-term use.

Still, people for whom taking hormonal birth control is unsafe should consult a doctor or pharmacist before taking Plan B.

No research has linked any deaths to the use of Plan B.

In the weeks after taking Plan B, light spotting between periods or a heavier flow are not typically a cause for concern. However, if these issues do not go away within a month or so, something other than Plan B may be responsible.

Heavy bleeding and intense pain can signal a serious underlying condition, such as endometriosis. Also, heavy bleeding can indicate an early miscarriage.

A healthcare provider should assess all heavy bleeding. If a person’s periods are irregular and heavy, taking additional hormonal contraceptives can help. If the bleeding results from pregnancy loss, the doctor may recommend surgery to remove remaining tissue.

If Plan B is ineffective, pregnancy can occur. A healthcare provider can describe further options, and it is best to consult them as soon as possible. If a woman decides to remain pregnant, seeking advice early can ensure that she receives the best prenatal care.

If a woman decides to have an abortion, it can involve taking a different kind of pill. Doctors refer to this as a medical abortion.The abortion pill can be effective up to 10 weeks after the woman’s last period.

Laws within the United States vary, and a woman may need to take the pill in the presence of a healthcare provider.

Another option is a surgical abortion. These tend to take place within the first trimester — within the first 12 weeks of pregnancy.

If a woman has a surgical abortion during the second trimester, the procedure may be more complicated and expensive.

If a woman in the U.S. wants to remain pregnant but is having trouble managing costs, Medicaid can help cover pregnancy care. To receive Medicaid, a woman must apply and receive approval.

See a doctor if:

  • changes in menstrual bleeding do not improve after 1–2 months
  • intense pain or very heavy bleeding occurs after taking Plan B
  • there are any signs of pregnancy, such as a missed period

Plan B is a method of emergency contraception — a person usually takes it after having unprotected sex. It contains a stronger dose of the synthetic hormone in many birth control pills.

It is a myth that Plan B causes an abortion or miscarriage. Also, most people who take the drug do not experience bleeding afterward.

That said, Plan B can cause temporary changes in the menstrual cycle because of hormonal disruptions.