While totally safe, one of Plan B’s main (albeit temporary) side effects is a change in your menstrual cycle. Many people find their period unpredictable or irregular for a few weeks after taking it. But if the change is ongoing (that is, it lasts for months on end), an underlying health condition could also be at play.

Read on to learn how the morning-after pill can affect your period, when things should return to your “normal,” and when it might be time to check in with your OB/GYN.

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How does Plan B affect your period?

After you take Plan B, your next period might be a little wonky. This relates to how emergency contraception works. “Levonorgestrel emergency contraceptive pills (commonly known as Plan B) prevent pregnancy by delaying ovulation,” says Mary Shorey, MD, an OB/GYN at Johns Hopkins Bayview Medical Center.

“In other words, Plan B prevents the body from releasing an egg…until sperm are no longer in the reproductive tract (usually a few days),” Dr. Shorey says. Long story short, if there’s no egg to fertilize by sperm, you can’t get pregnant.

But delaying ovulation can affect your period. During the average menstrual cycle, your period comes about two weeks after you ovulate (if the egg hasn’t been fertilized). “Because Plan B delays ovulation by a few days, it also may delay your period by a few days,” Dr. Shorey says. On the other hand, some people get their period earlier than usual, according to Planned Parenthood.

Lighter or heavier bleeding than normal (i.e., what you’re used to) is possible, too. It’s also common to have spotting or “irregular bleeding in the week, or even month, following emergency contraception use,” says Sonya Brar, MD, an OB/GYN and instructor in the department of obstetrics, gynecology and reproductive science at the Icahn School of Medicine at Mount Sinai. In fact, the earlier you take Plan B in your menstrual cycle, “the more likely you are to have irregular bleeding that month,” Dr. Brar says.

If you take Plan B more than once a month (more on this later), these effects can be magnified. Meaning your period might be even more unpredictable (think: more irregular bleeding or longer delays in getting your period), Dr. Shorey adds.

Luckily, all these side effects are short-lived (they usually subside on their own within a few weeks) and don’t require treatment.

Other possible side effects of Plan B

Taking plan B can temporarily mess up your usual menstrual cycle. But it can also have other possible, minor side effects. These include the following, per Planned Parenthood and Mount Sinai:

• Headache
• Upset stomach, nausea, or vomiting
• Lightheadedness or dizziness
• Tender breasts
• Fatigue

Like period symptoms, these side effects might be annoying and inconvenient, but they should only last a short time. Keep in mind, though, if you have nausea and vomit within two hours of taking the pill, it won’t be effective, according to Planned Parenthood.

Plan B spotting vs. implantation bleeding

Light spotting is a common side effect of Plan B. But it’s also a possible symptom of implantation bleeding—i.e., the type of spotty bleeding that happens one to two weeks after fertilization (when an embryo embeds into the uterine lining), Dr. Brar says. So, how can you tell the difference between the two?

“That can be tricky,” Dr. Brar says. Implantation bleeding “is often accompanied by other symptoms such as cramping, nausea, and breast tenderness,” she says. But as we learned, Plan B can have some of these side effects as well.

Dr. Shorey agrees: “It’s very hard, if not impossible, to tell the difference” between Plan B spotting and implantation bleeding. The best way to rule out a pregnancy? Take a pregnancy test if you don’t get your period within three weeks after taking Plan B, or if it doesn’t come within a week of when you’d normally expect it, she says.

Is it normal for Plan B to mess up your cycle for months?

The short answer? No. “It is not normal for Plan B to alter your menstrual cycle for months,” Dr. Shorey says. “Most people will get their period within one week of the expected time,” she adds.

Dr. Brar seconds this: “Your period should return to normal either within that same cycle or after a month.”

But if your cycle seems off months after taking Plan B, you might be wondering what gives? “Many aspects of your medical history may also play a role in your period timing,” Dr. Shorey says. “Different conditions, like polycystic ovary syndrome (PCOS), uterine polyps or fibroids, or thyroid abnormalities, can affect your periods,” she says. Endometriosis and other hormonal imbalances can also mess with your menstrual cycle, Dr. Brar adds.

“Similarly, some medications, like some types of birth control or blood thinners, can change your bleeding patterns or alter when your period comes,” Dr. Shorey says.

“If your period is delayed by more than a week, or if you have any other symptoms like lower abdominal pain, you should call your doctor…to rule out pregnancy, and other conditions that can cause irregular bleeding,” Dr. Brar says.

When to see a doctor

Normally, you don’t need to see your doctor after taking Plan B. But if your period doesn’t come within three weeks of using the morning-after pill, it might be time to check in with your doctor (and take a pregnancy test).

“You should also see a doctor if you have lower abdominal pain or persistent irregular bleeding,” Dr. Shorey says. “These can be signs of ectopic pregnancy [when a fertilized egg has implanted outside the uterus] or other conditions,” she says.

This might also be a good time to chat with your doctor about prescribing Plan B for the future. While the morning-after pill is available over the counter, “many people still have difficulty finding or purchasing it at their pharmacy when they need it,” Dr. Shorey says. “Your doctor can typically prescribe it for you to help reduce the cost and ensure its availability. They can even do this in advance so that it’s ready for you ahead of time, if you need it,” she says.

While you’re at the office, you should also take the opportunity “to discuss your options for birth control, whether hormonal or non-hormonal, to find one that’s right for you,” Dr. Brar adds. “Plan B is not meant to be a regular form of birth control because it’s not as effective,” she says.

FAQ

Can Plan B cause two periods in a month?

“Sometimes Plan B can cause irregular bleeding or spotting,” Dr. Shorey says. “This can make it seem like you have two periods in a month depending on the heaviness of the bleeding.” Bear in mind, “if you take Plan B multiple times within one month, you may be more likely to have these irregular bleeding episodes,” she adds.

Spotting or irregular bleeding usually resolves after a month, but if it persists, “it is always a good idea to check in with your OB/GYN to figure out what might be causing it,” Dr. Shorey says.

How many times can you take Plan B in a month?

Technically, you can take Plan B as many times as you need (it’s not harmful), but it shouldn’t substitute for regular routine birth control, according to the Cleveland Clinic. That’s because it doesn’t work as well as other forms of birth control, Dr. Brar says. And it may be less effective for certain groups, such as people with obesity, Dr. Shorey adds. Plus, another thing to consider? Plan B won’t protect you against sexually transmitted infections.

At the end of the day, it’s probably best to stick to taking Plan B to prevent pregnancy in emergency situations—i.e., when you didn’t use birth control (or it failed) or in cases of non-consensual sexual intercourse.

What is the maximum delay in periods if you’re not pregnant?

Once you use the morning-after pill, your period should come within one week of when it was supposed to happen, Dr. Shorey says. If it “does not return to normal within three to four weeks after taking Plan B, you should take a pregnancy test and make an appointment to see an OB/GYN,” she says.

Can you take Plan B if you’re nursing?

While your risk of getting pregnant if you exclusively breastfeed, chestfeed, or pump (i.e., you only feed your baby your milk, not formula or other solid foods) is lower, it can still happen. So if you had unprotected sex, and you don’t wish to conceive again, you might wonder if it’s okay to take the morning-after pill. Rest assured, “it is safe to take when breastfeeding,” Dr. Brar says. “It can, however, temporarily lower milk supply,” she adds, so keep this in mind when taking it.