Plan B is an emergency contraceptive designed to prevent pregnancy after unprotected sex or a known contraceptive failure. It works best when taken as soon as possible, and it must be taken within 72 hours (3 days) of the event. The sooner you take it, the more effective it is at preventing ovulation and stopping pregnancy before it starts.
When Exactly Should You Take Plan B?
Take Plan B as soon as you can after unprotected sex. The official window is 72 hours, but effectiveness drops noticeably with each passing day. Taking it within the first 24 hours gives you the best chance of preventing pregnancy.
Plan B does not end a pregnancy that has already implanted. It works by delaying or stopping the release of an egg from the ovary. If ovulation has already happened, Plan B may not work. This is why timing matters so much.
If you are already ovulating when you take it, the medication cannot stop an egg that has already been released. Sperm can survive inside the female reproductive tract for up to 5 days, which is why emergency contraception must be taken quickly after unprotected sex.
How Does Plan B Work in the Body?
Plan B contains levonorgestrel, a synthetic hormone similar to the progesterone your body produces naturally. The pill works by interfering with the normal hormonal signals that trigger ovulation.
When taken before ovulation, the hormone thickens cervical mucus. This makes it harder for sperm to travel through the cervix and reach an egg. It also alters the lining of the uterus, although research shows this effect is not the primary way the medication works.
The key mechanism is preventing ovulation. If no egg is released, there is nothing for sperm to fertilize. Once fertilization has occurred and the egg has implanted in the uterus, Plan B has no effect. It is not an abortion pill.
What If You Are Already Ovulating When You Take It?
This is the most important limitation of Plan B. If you take the pill after your ovary has already released an egg, it cannot stop that egg from being fertilized. The medication simply has no target to act on.
This is why some women get pregnant despite taking Plan B correctly. The failure rate is not because the pill was taken wrong. It is because ovulation had already occurred before the pill was taken.
Most women cannot know exactly when they are ovulating. Menstrual tracking apps can estimate, but they are not precise. If you have a regular cycle, ovulation typically happens around day 14, but this varies from woman to woman and cycle to cycle.
Some research suggests that Plan B may be less effective in women who weigh more than 165 pounds (75 kg). The medication appears to work less reliably at higher body weights. If you weigh more than this, talk to a pharmacist or doctor about alternative emergency contraception options like ella or a copper IUD.
How Effective Is Plan B at Different Time Points?
Effectiveness is directly tied to how quickly you take the pill after unprotected sex. The numbers below reflect what clinical research has consistently shown.
- Within 24 hours: Most effective. Studies show it prevents pregnancy in the vast majority of cases when taken this quickly.
- Within 72 hours: Still effective, but the failure rate increases with each passing day.
- After 72 hours: Effectiveness drops significantly. It may still have some effect, but it is no longer considered a reliable emergency contraceptive.
No emergency contraceptive is 100% effective. Even when taken within the first 24 hours, there is a small chance of pregnancy. This is not a failure of the medication. It reflects the biological reality that ovulation timing cannot always be predicted.
What Counts as Unprotected Sex for Plan B Purposes?
Plan B is intended for situations where contraception failed or was not used. Common scenarios include a broken condom, a missed birth control pill, or sex without any contraceptive method.
If a condom slipped off or broke, Plan B is appropriate. If you missed two or more birth control pills in a row and had sex, Plan B may be recommended depending on where you are in your cycle. If you use a diaphragm or cervical cap and it shifted out of place, Plan B is a reasonable option.
Plan B is also used after sexual assault when no contraceptive was used. In these situations, the same 72-hour window applies.
It is not intended for regular use as a primary birth control method. Plan B contains a much higher dose of hormones than daily birth control pills. Using it repeatedly can disrupt your cycle and cause irregular bleeding, though it does not cause long-term fertility problems.
What Are the Side Effects of Plan B?
Most women tolerate Plan B well, but side effects are common. Nausea is the most frequently reported side effect. Some women vomit after taking it, which can reduce effectiveness. If you vomit within 2 hours of taking the pill, contact a pharmacist or doctor about whether you need a second dose.
Other common side effects include headache, dizziness, breast tenderness, and fatigue. These typically resolve within a day or two.
Your next period may come earlier or later than expected. It may be heavier or lighter than usual. If your period is more than a week late, take a pregnancy test. This is the standard advice regardless of whether you have symptoms.
Spotting between periods is also common in the weeks after taking Plan B. This is a hormonal effect and does not mean the medication failed or that something is wrong.
Are There Alternatives to Plan B?
Plan B is not the only emergency contraceptive option. Knowing your alternatives matters because Plan B has limitations, particularly around timing and body weight.
ella (ulipristal acetate) is a prescription emergency contraceptive that works up to 120 hours (5 days) after unprotected sex. Research indicates it remains effective throughout that entire window, whereas Plan B loses effectiveness over time. It is also more effective than Plan B in women who weigh more than 165 pounds.
The copper IUD (Paragard) is the most effective emergency contraceptive available. When inserted within 5 days of unprotected sex, it prevents pregnancy in more than 99% of cases. It also provides ongoing contraception for up to 10 years. Any doctor or clinic that inserts IUDs can place it for emergency contraception.
The copper IUD works differently from hormonal emergency contraceptives. It creates an environment in the uterus that is toxic to sperm and eggs, preventing fertilization. It works regardless of where you are in your cycle and is not affected by body weight.
If you are considering emergency contraception and have any concerns about timing or your weight, ask a healthcare provider about ella or the copper IUD. These options are more effective in situations where Plan B may not be reliable.
What Should You Do If Plan B Fails?
If your period is more than a week late after taking Plan B, take a home pregnancy test. These tests are highly accurate when taken after a missed period.
If the test is positive, contact a healthcare provider to confirm the pregnancy and discuss your options. Plan B does not harm a developing fetus if it fails and pregnancy occurs. There is no evidence that taking levonorgestrel emergency contraception causes birth defects.
Some women experience light bleeding after taking Plan B and mistake it for a period. This is not a reliable sign that the medication worked. The only way to confirm you are not pregnant is a pregnancy test after your expected period date.
Can You Take Plan B While Breastfeeding?
Plan B is considered safe to take while breastfeeding. The hormone passes into breast milk in small amounts, but research has not found harmful effects on nursing infants.
Some clinicians suggest pumping and discarding breast milk for a few hours after taking the pill, but this is not a formal medical requirement. The American College of Obstetricians and Gynecologists considers levonorgestrel emergency contraception compatible with breastfeeding.
If you have concerns, talk to your obstetrician or a lactation consultant. They can give you personalized guidance based on your situation.
Does Plan B Affect Future Fertility?
No. Taking Plan B does not affect your ability to get pregnant in the future. The hormones are cleared from your body within a few days, and your normal cycle resumes.
Some women worry that using emergency contraception repeatedly will cause permanent changes to their cycle. This is not supported by evidence. Your cycle may be irregular for one or two months after taking it, but it will return to normal.
Repeated use is not recommended as a birth control strategy, but this is about effectiveness and cost, not safety. Plan B is less effective than regular birth control methods when used repeatedly, and it is significantly more expensive per use.
Frequently Asked Questions
Can I take Plan B after 72 hours?
Plan B is officially approved for use within 72 hours, and effectiveness drops significantly after that point. If more than 3 days have passed, ask a healthcare provider about ella or a copper IUD, which work up to 5 days.
Will Plan B work if I am already ovulating?
No. Plan B works by preventing ovulation, so it cannot stop pregnancy if an egg has already been released. This is why taking it as early as possible matters.
How many times can I take Plan B in one cycle?
There is no medical limit on how often you can take Plan B, but it is not designed for repeated use. Frequent use causes irregular bleeding and is less effective than regular birth control methods.
Does Plan B make you gain weight?
No clinical evidence shows that a single dose of Plan B causes long-term weight gain. Any temporary bloating or water retention typically resolves within a few days.
