Plan B (levonorgestrel) is an emergency contraceptive pill that works by temporarily halting ovulation.

Plan B is sometimes referred to as the morning-after pill. However, if you’ve had unprotected penis-in-vagina sex at night, don’t wait until morning to take it. The sooner you take Plan B, the better your chances are of avoiding pregnancy.

In fact, one manufacturer of the medication recommends taking it within 12 hours, if possible. They also state that Plan B is 95 percent effective when used within 24 hours.

Plan B can’t stop or reverse ovulation that has already occurred. It also can’t stop an established pregnancy from progressing. For those reasons, Plan B is most effective when taken as quickly as possible.

Sperm can live for up to 5 days within the female reproductive tract. If you ovulate and release an egg, the egg can become fertilized at any point during that timeframe.

Even if you have regular periods, you might not be able to gauge exactly when ovulation is going to occur. For that reason, taking Plan B before ovulation, and as quickly as possible after unprotected sex, is your best bet.

Plan B is approved by the Food and Drug Administration (FDA) for use within 72 hours (or 3 days) of having unprotected sex.

Plan B (levonorgestrel) is an emergency contraceptive pill that may prevent pregnancy after you’ve had unprotected penis in vagina sex.

It can be used to prevent pregnancy after contraception failure, such as having a condom break during sex. It can also be used as emergency contraception by people who miss one or more doses of their regular birth control medication.

Brand name products for levonorgestrel are:

  • Aftera
  • AfterPill
  • EContra
  • My Choice
  • My Way
  • Next Choice
  • One Dose
  • One Step
  • Option 2
  • Plan B
  • Plan B One-Step
  • Preventeza
  • Take Action

The only active ingredient in Plan B is levonorgestrel. Levonorgestrel is a synthetic-steroid progestin hormone that mimics progesterone.

Progesterone is a naturally occurring hormone that’s produced by your body after ovulation has taken place. When progesterone levels are high, ovulation cannot occur.

Taking Plan B tricks your body into thinking you’ve already ovulated. This temporarily stops your ovary from releasing an egg.

There are a few reasons Plan B may not work for you including:


Research from 2017 suggested that Plan B may be less effective in people who weigh over 155 lbs. or have a body mass index (BMI) >30.

However, the FDA has not changed its recommendations about Plan B for people in this weight category.

Vomiting after taking

Vomiting is an occasional side effect of Plan B. If you vomit within 2 hours of taking it, your system might not have had an opportunity to absorb the medication completely.

If this occurs, call your doctor. You may need another dose of Plan B.

More than 3 days

If more than 3 days have passed since you had unprotected sex, Plan B should not be your first-line emergency contraception choice.

There are other emergency contraceptives that may be more effective if too much time has passed to take Plan B. These include:

  • ella. ella (ulipristal) is another type of emergency contraception pill. It works up to 5 days after having unprotected sex. ella is only available by prescription and should not be taken in conjunction with Plan B.
  • ParaGard (copper intrauterine device). The ParaGard copper intrauterine device (IUD) can be used as emergency contraception within 5 days of having unprotected sex. The copper in the IUD reduces sperm mobility, making it almost impossible for a sperm to fertilize an egg. An IUD must be inserted by a medical professional. It can remain in place, preventing pregnancy, for up to 10 years.

Plan B is an emergency contraceptive pill. It’s meant to be taken within 72 hours (or 3 days) of having unprotected penis-in-vagina sex. It’s best to take Plan B as quickly as possible. You don’t need a prescription or proof of age to purchase Plan B.

Plan B is not an abortion pill. It won’t terminate a pregnancy that has already been established.