• Plan B can affect your period for one menstrual cycle and alter when your period comes, menstrual flow, and how long your period lasts.
• Plan B can often alter when your period comes or change its flow. This is due to the active hormone in Plan B called levonorgestrel that prevents pregnancy. 
• Plan B does not terminate pregnancy and should not be taken as regular birth control.
• Visit Insider’s Health Reference library for more advice.

Plan B is a form of emergency birth control, often referred to as the morning-after pill, that can help prevent pregnancy in the event of unprotected sex or birth control failure. 

Taking Plan B could affect the timing of your next period or cause other menstrual irregularities like bleeding between periods, and heavier or lighter menstrual flow. However, plan B’s effects on your period shouldn’t persist beyond one menstrual cycle.

Plan B should only be used as a backup in case you need emergency birth control. It should not be taken regularly or used as a replacement for other forms of birth control.

Here’s how plan B may disrupt your menstrual cycle and what to expect after taking it.

What is in Plan B? 

"Levonorgestrel is the active hormone in Plan B," says Krishna Kakani, an MD who has her own OB/GYN practice. 

Levonorgestrel is a synthetic progestin hormone that prevents pregnancy in two ways: 

1. By delaying the release of the egg from the ovary. 
2. In case the egg has already been released, it works by blocking fertilization and altering the uterine lining so that the egg cannot attach itself to the wall of the uterus. 

It's important to note that plan B works to prevent fertilization, it will not terminate a pregnancy if the egg has already implanted. 

Since levonorgestrel affects ovulation and the uterine lining to prevent pregnancy, taking Plan B could alter the timing, length, and flow of your next period. 

"Emergency contraceptives that contain levonorgestrel are most effective if taken within 72 hours of unprotected sex," says Kakani. Plan B is just one type of emergency contraceptive that contains levonorgestrel. Others brands include: 

• Next Choice 
• My Way 
• Option 2 
• Fallback 
• Opcicon 

Levonorgestrel is also an ingredient in other forms of birth control like oral birth control pills, vaginal rings, contraceptive implants, and birth control patches. 

Ella, another emergency contraceptive, contains a different hormone, known as ulipristal acetate. Like levonorgestrel, it works by preventing the release of the egg from the ovary. However, unlike levonorgestrel, which only offers you a 72-hour window, ella offers you a 120-hour window to prevent pregnancy after having unprotected sex. 

How does Plan B affect your cycle? 

A 2006 study investigated how emergency contraceptives containing levonorgestrel affect menstruation. The study found that approximately 15% of women experienced disruptions in their menstrual cycle after taking the medication. The women reported some or multiple of the following side effects: 

• Delayed or early periods 
• Bleeding between periods
• Shorter or longer periods 
• Heavier or lighter menstrual flow

You can take Plan B at any point in your menstruation cycle. However, the side effects you experience could vary depending on where you are in your cycle. For instance, a small 2005 study found that participants taking Plan B in the first few weeks of their menstrual cycle  caused their period to arrive earlier than usual; whereas taking it in the last week of their cycle delayed their period.

Apart from affecting your menstrual cycle, taking Plan B can also cause short-term side effects that may start when you take the pill and persist for a few days. Side effects may include: 

• Nausea 
• Vomiting 
• Diarrhea 
• Pelvic or abdominal pain 
• Fatigue 
• Dizziness 
• Headache
• Breast pain or tenderness 

Plan B is typically administered as a single tablet. However, you may need a second dose if you vomit within two hours of taking the pill, as it could mean that the pill hasn't been absorbed and is therefore ineffective in preventing pregnancy. If this is the case, contact your doctor and see if you should take a second dose. 

It is not recommended to resume sexual intercourse without another form of contraception, as Plan B does not provide lasting protection against pregnancy and is not intended to replace other forms of birth control. However, if you have unprotected sex again, you can take Plan B if you need to. 

In fact, there's no limit on how many times you can take plan B in one cycle. While it could cause short-term side effects and affect your next period, it doesn't affect your fertility in the long-run. It is just not as effective as regular birth control methods that are taken correctly.

Research shows that the side effects of Ella are similar to those of emergency contraceptives containing levonorgestrel, like Plan B. However, you need to wait five days after taking Ella to resume other hormonal contraceptives like birth control pills, rings, patches, implants, or injections.

If your cycle is altered by Plan B, how will you know that you're not pregnant? 

Experiencing vaginal bleeding after taking Plan B could be a sign that you aren't pregnant. "You may experience bleeding similar to a period after taking Plan B, which may indicate that fertilization has not occurred," notes Kakani.

Your cycle should also go back to normal soon. Most of the changes caused by emergency contraceptives like Plan B do not persist beyond one menstrual cycle. 

If your period is over a week late after you've taken Plan B, there's a chance that you may be pregnant, so consider taking a pregnancy test. 

Insider's takeaway

Plan B is an effective form of emergency contraception that you can take up to 72 hours after having unprotected sex. It delays ovulation and prevents fertilization, which may cause side effects like nausea and vomiting, as well as disruptions to your next menstrual cycle. 

Plan B should therefore be taken only in emergencies, if you've had unprotected sex or if your regular birth control method has failed. It's not recommended to take Plan B frequently, or as a substitute for .