Plan B, also known as the morning-after pill, is a type of emergency contraception. It can help prevent pregnancy if you take it soon after having penis-in-vagina sex without a condom.

Plan B isn’t regular contraception, so you should use it only in emergency situations, such as:

  • if you had penis-in-vagina sex without a condom or another barrier method
  • if the condom broke during penis-in-vagina sex
  • if you forgot to take your birth control pill the day before or after penis-in-vagina sex

Plan B contains levonorgestrel, a synthetic version of the hormone progesterone. This synthetic hormone is also found in combination pills and other forms of hormonal birth control. But Plan B contains a much larger dose.

This surge of levonorgestrel mimics the effects of progesterone in your body, temporarily delaying the release of an egg from your ovary. Ovulation is necessary for fertilization and implantation to occur.

If your ovary does not release an egg, there’s nothing for sperm to fertilize, meaning pregnancy isn’t possible. Plan B works only to prevent a pregnancy — it can’t end one.

Plan B and other levonorgestrel morning-after pills can reduce the chance of pregnancy by 75% to 89% if taken within 72 hours (3 days) after sex. The sooner you take the pill, the more effective it is.

Some medications and supplements can make Plan B less effective. Consult a healthcare professional before taking Plan B if you’re currently taking any of the following:

You can take Plan B up to 120 hours (5 days) after penis-in-vagina sex, but it’s most effective when taken within 72 hours (3 days).

There’s no official weight limit for Plan B. However, older research suggests that levonorgestrel morning-after pills may be less effective in people who weigh 165 pounds or more or have a BMI of 30 or more.

Due to conflicting results, some researchers have recommended additional counseling for people who fall into either of those categories. But they point out that there’s no reason not to take Plan B.

It can still help prevent pregnancy, so if Plan B is the only emergency contraceptive available to you, you can still benefit from taking it.

More research is needed to better understand how weight and BMI may change the drug’s effectiveness.

Plan B and other levonorgestrel morning-after pills are considered safe. The medication stays in your body for only a few days.

Possible temporary side effects include:

  • spotting or vaginal bleeding outside of menstruation
  • shorter- or longer-than-usual menstrual cycle
  • fatigue
  • chest tenderness
  • nausea
  • headache
  • vomiting
  • abdominal pain
  • dizziness

If you vomit within 2 hours of taking this medication, consult a healthcare professional to determine whether you need to take a second dose.

Remember that the biggest risk is that the morning-after pill might not work. So, if your period or period-like withdrawal bleeding is significantly late, take a pregnancy test.

Brand-name medications such as Plan B are usually more expensive than their generic counterparts. Plan B typically costs $40 to $50 per pill, while generic levonorgestrel pills cost less and work in the exact same way.

There are also other ways to reduce the cost. For example, if you have insurance or Medicaid, you may be able to get Plan B for free with a doctor’s prescription.

You may be able to get Plan B for free or at a reduced cost at your local Planned Parenthood, health department, or community clinic.

You can also clip a coupon from the “Ready to save on Plan B?” section on the manufacturer’s website. Be sure to take note of any restrictions or requirements for using the coupon.

Plan B is available without a prescription at most drugstores and pharmacies. There is no age limit, and you don’t have to show an ID to buy it.

Depending on the store, Plan B may be stocked in the family planning or sexual wellness aisle, stored behind the counter in the pharmacy, or stored behind the counter at checkout.

If you’d rather buy it online, you can order the medication directly from the manufacturer or through the website of your preferred retailer. But it may take up to a week for the medication to arrive.

Plan B is most effective if taken within 72 hours (3 days). It works only if you have not already ovulated during your current menstrual cycle.

If your period or expected period-like bleeding is more than a week late after taking Plan B, take a pregnancy test and follow up with a healthcare professional.

Ella is an ulipristal acetate pill that delays ovulation. If taken within 120 hours (5 days) after sex, it can reduce the chance of pregnancy by 85%.

Ella may be more effective than Plan B for people who weigh 165 to 194 pounds. However, ella may not be as effective for people who weigh 195 pounds or more.

Unlike Plan B, ella requires a prescription.

You can take Plan B and other levonorgestrel pills as often as you need them. You can only take ella once per menstrual cycle. There are no long-term side effects or risks with either form of emergency contraception.

Avoid taking a levonorgestrel pill and ella within 5 days of each other. The drugs may counteract each other.

Plan B is not an “abortion pill.” Levonorgestrel works to prevent pregnancy before it takes place by preventing ovulation. It won’t have an effect if you’re already pregnant.

Plan B and other levonorgestrel morning-after pills must be taken within 72 hours (3 days) for maximum benefit. The sooner you take the medication, the more effective it will be.

Plan B can affect the timing of your next period or period-like withdrawal bleeding. So, if you haven’t experienced a period as expected within 3 weeks of taking Plan B, consider taking a pregnancy test.

If you get a positive result, talk with a healthcare professional for advice on the options that are available to you.