Emergency contraception can be a safe way to prevent an unwanted pregnancy after unprotected sex. While there is a variety of methods, one of the most well known is the Plan B One-Step pill.

Plan B is an emergency contraceptive also known as the "morning-after" pill. It contains a hormone called levonorgestrel which can block ovulation, stop fertilization, or prevent the fertilized egg from attaching to the wall of your uterus after unprotected sex. 

There are other brands of levonorgestrel pills such as My Way, Next Choice, and EContra One-Step, all of which work the same as Plan B. 

Here are seven things you should know before taking plan B: 

1. Your weight may make Plan B less effective 

If you weigh more than 155 lbs or have a BMI over 25, Plan B may be less effective for you. 

A large 2014 study found pregnancy increased significantly by 5% to 6% in people who were overweight or obese. Another 2015 study found levonorgestrel was less effective in participants with a BMI greater than 25 than other emergency contraceptives, such as ulipristal acetate (Ella) or a copper IUD. 

However, a large 2017 pooled analysis of four randomized trials completed by the World Health Organization found pregnancy was low across all BMIs after taking levonorgestrel and only one trial showed a decrease in efficacy for obese women. 

"There are also alternative emergency contraception methods that will be more effective for all weights," says Eliza Bennet, MD, an OB-GYN at the University of Wisconsin Health. "But if the only option is Plan B, then it's always best to take what might be more effective than nothing at all." 

2. The sooner you take Plan B, the more effective it is

Plan B can be taken up to three days (72 hours) from when you had unprotected sex, but it is most effective to take the pill as soon as possible. The pill will decrease in efficacy from day one to day three. 

If you take the pill 12 to 24 hours after having unprotected sex, it's around 95% effective, but if you take it 48 to 72 hours after, it will decrease to 61%. Since Plan B works to prevent ovulation, if you have already ovulated, then the pill will not work. 

"It really matters where a person is in their menstrual cycle," says Rebecca Jacobson, MD, an OB-GYN at Northshore Medical Group. "If you have unprotected sex right at the time of ovulation and then take Plan B two days after, it will be less effective than if you took it right away." 

3. Plan B can cause irregular periods 

It's perfectly normal to have irregular bleeding after you take Plan B. The pill interrupts the regular hormonal cyclical flow of your period, says Bennett, so it may take your body a bit longer to realize it needs to begin menstruation. 

Your period may be delayed three to five days, or you may experience your period earlier than usual. You may also have some spotting as well, and a lighter or heavier flow. 

4. Plan B does not impact your fertility

One common misconception about Plan B, or any emergency contraception, is that the pill will affect your fertility. 

"There is no evidence at all that repeatedly taking Plan B can impact your fertility," says Lee Roosevelt, PHD, MPH, a certified nurse-midwife and clinical assistant professor at the University of Michigan. "In fact, [Levonorgestrel] is a progesterone, which if you speak with a reproductive endocrinologist, is one of the things they look for in maintaining a pregnancy is ensuring you have adequate progesterone levels." 

Plan B will also not increase your chances of miscarriages or impact your chances of becoming pregnant in the future. 

5. Plan B is not an abortion pill

Another common misconception about Plan B is that the pill will abort a pregnancy. This is not true. Plan B will only prevent ovulation from occurring and has no effect on an already started pregnancy. 

Abortion pills, also called a medication abortion, are two pills taken 24 to 48 hours apart after an ultrasound up to 70 days after unprotected sex. 

6. Plan B can expire

Plan B has a shelf life of four years. Taking the pill after the expiration date is not dangerous, according to Bennett, but the hormones may lose potency over time, meaning its less effective at preventing pregnancy. 

"I would say try to take an unexpired pill pack, but if all you have is an expired pill pack, then go ahead and take it," says Bennett. 

7. There are other types of emergency contraception

There are other options for emergency contraception that may be better for you. A copper IUD and the 'day after' pill called Ella are both non-hormonal and can last up to five days (120 hours) after unprotected sex. Ella has a weight limit of 190 lbs while the copper IUD works at all weights. 

However, Ella does need a prescription and the copper IUD must be inserted by a medical professional. So, some people might choose Plan B and other levonorgestrel pills for quick availability and easy access. 

Insider's takeaway

Plan B can be a safe and effective way to prevent an unintended pregnancy for up to 72 hours after unprotected sex. It's best to take the pill as soon as possible for the best results, but it may be less effective for people who are over a BMI of 25. There may also be some brief irregularities in your period after taking the pill, but Plan B does not cause abortion or impact your fertility. 

If Plan B doesn't feel right for you, there are other types of emergency contraception, such as Ella or a copper IUD, which are more effective and can last up to five days. But if you have any questions about which emergency contraception is best for your body, you can always ask your doctor or another medical professional beforehand. 

Related stories from Health Reference:

• Birth control pills can help treat acne, but some types are better than others
• The best diet for endometriosis that can help relieve painful symptoms
• How to treat vaginismus through dilation training and counseling
• You can get pregnant with an IUD but it's extremely rare
• How long it takes for condoms, birth control pills, and other types of birth control to expire

Join the conversation about this story »