Plan B (levonorgestrel) is an emergency contraceptive pill designed to prevent pregnancy after unprotected sex or birth control failure. It works for up to 72 hours (3 days) after the event, but it is most effective the sooner you take it. The drug itself leaves your body within a few days, though it can cause menstrual cycle changes that last longer.
How Long Does Plan B Stay Effective in Your Body?
Plan B is most effective when taken within 72 hours of unprotected sex. Some research suggests it may offer some protection up to 120 hours (5 days) after, but the evidence for that extended window is less consistent. The official labeling and most clinical guidance point to the 72-hour mark as the reliable window.
Effectiveness is not a straight line. Taking it within the first 24 hours gives the highest chance of preventing pregnancy. Each passing day reduces effectiveness. This is why the instruction to take it as soon as possible is not marketing language — it reflects how the medication works.
What Happens Inside Your Body After You Take It
Plan B contains a synthetic hormone called levonorgestrel. It works primarily by delaying ovulation. If ovulation has not happened yet, the pill pushes it back long enough that sperm in the reproductive tract die off before an egg is released.
If ovulation has already occurred, Plan B does not work. It does not end an existing pregnancy, and it does not interfere with an implanted embryo. This is an important distinction — Plan B is not an abortion pill. It simply gives your body more time before releasing an egg.
The hormone is processed by your liver and kidneys. Most of it is eliminated within a few days. A single dose of 1.5 mg levonorgestrel has a half-life of roughly 24 to 32 hours, meaning it takes about a day for your body to clear half the dose. After about 5 days, the drug is essentially gone.
How Long Do Side Effects Last?
Most side effects are short-lived and resolve within 24 to 48 hours. Nausea is the most common complaint. Some people vomit after taking it — if this happens within 2 hours of taking the pill, contact a clinician because a repeat dose may be needed.
Other common side effects include:
- Headache
- Dizziness
- Breast tenderness
- Fatigue
- Lower abdominal pain
These symptoms typically fade within a day or two. If they persist beyond a few days or become severe, that is a reason to seek medical advice — not necessarily because of the pill itself, but because the symptoms could indicate something else.
How Long Until Your Period Returns to Normal?
The most noticeable effect of Plan B is on your menstrual cycle. Because the hormone delays ovulation, your next period will likely come later than expected. This is normal.
Most people get their period within one week of the expected date. Some people experience it up to a week early; others up to a week late. The timing depends on where you were in your cycle when you took the pill.
Your period after taking Plan B may be lighter, heavier, or different in texture than usual. Spotting between periods is also common in the weeks following use. These changes usually resolve by the next cycle. If your period is more than 7 days late, take a pregnancy test.
Does Plan B Work If You Are Overweight?
This is where the evidence gets more complicated. Some studies suggest that levonorgestrel emergency contraception is less effective in people with a body mass index (BMI) over 25, and significantly less effective in those with a BMI over 30. The exact threshold is debated, and research results vary.
This does not mean the pill does nothing for people in this weight range. It means the failure rate is higher. An alternative emergency contraceptive called ulipristal acetate (ella) may be more effective for people with higher BMI, but it requires a prescription and is less widely available.
If you are concerned about weight and Plan B effectiveness, speak with a pharmacist or clinician before taking it. They can discuss options based on your situation. This is not a topic to guess about.
How Soon Can You Take a Pregnancy Test?
A pregnancy test taken immediately after taking Plan B will not tell you anything useful. The test measures a hormone called hCG that only appears after implantation, which typically happens 6 to 12 days after fertilization.
The most reliable time to test is 14 days after unprotected sex. Testing at this point gives an accurate result for nearly all pregnancies. A test at 21 days after unprotected sex is considered definitive.
If your period is late — more than 7 days past when you expected it — take a test regardless of when you had sex. This is the clearest sign that something needs checking.
Can You Take Plan B More Than Once in a Cycle?
Yes, you can take Plan B more than once in the same menstrual cycle. There is no medical limit on how often you can use it. However, it is not designed for repeated use.
Taking Plan B multiple times in one cycle increases the likelihood of cycle disruption. Your period may be significantly delayed or irregular. It also exposes you to a large dose of hormones repeatedly, which some people find unpleasant.
Plan B is not birth control. It is a backup option. If you find yourself needing it frequently, talk to a clinician about a regular contraceptive method that is more effective and easier to maintain.
What Are the Alternatives to Plan B?
The copper IUD is the most effective form of emergency contraception. It can be inserted up to 5 days after unprotected sex and is more than 99% effective. It also provides ongoing birth control for up to 10 years. This is the best option if you want long-term protection and can see a clinician quickly.
Ulipristal acetate (ella) is a prescription emergency contraceptive that is effective up to 5 days after unprotected sex. Some studies suggest it is more effective than Plan B, especially later in the 5-day window. It requires a doctor’s visit or a telehealth appointment.
Plan B is the most accessible option because it is available over the counter without age restrictions or a prescription. For many people, that accessibility matters more than the small differences in effectiveness between methods.
When Should You See a Doctor?
Seek medical attention if you vomit within 2 hours of taking Plan B — you may need a second dose. Also see a clinician if you experience severe abdominal pain, especially on one side, which could indicate an ectopic pregnancy.
If your period is more than 7 days late after taking Plan B, take a pregnancy test. If it is positive, contact a clinician. If you have irregular bleeding that continues for several weeks, that also warrants a check-up.
These situations are uncommon, but they are not rare enough to ignore. Trust your body. If something feels off, get it checked.
Frequently Asked Questions
How long does Plan B stay in your system?
The active hormone is mostly cleared from your body within 5 days. Side effects like nausea and headache typically resolve within 48 hours.
How late can your period be after taking Plan B?
Your period can arrive up to a week late or a week early. If it is more than 7 days late, take a pregnancy test.
Can Plan B fail even if taken within 24 hours?
Yes, it can fail. No emergency contraceptive is 100% effective. If you ovulated before taking the pill, it will not prevent pregnancy.
Is Plan B the same as the abortion pill?
No. Plan B prevents ovulation and does not end an existing pregnancy. The abortion pill is a different medication used after pregnancy is confirmed.
