Plan B is a form of emergency contraception. It is not a daily pill. You take it only once, within a specific window after unprotected sex. The pill is designed for a single use per incident, not for ongoing birth control.
The standard dose is one pill taken as soon as possible after unprotected sex. You can take it up to 72 hours afterward, but it works best the sooner you take it. Taking it later in that window reduces its effectiveness, though it can still work.
How Long Do You Take Plan B Before It Works?
Plan B is a one-time dose. You swallow a single pill. There is no multi-day course of treatment like antibiotics.
The active ingredient is levonorgestrel, a synthetic hormone. It works by delaying ovulation. If ovulation has already happened, the pill will not work. That is why timing matters so much.
The pill is most effective when taken within the first 24 hours. Effectiveness gradually declines each day after that. By the third day, it is significantly less reliable, though still better than doing nothing.
You do not take another dose the next day. One pill is the entire treatment. If you have sex again after taking Plan B, you need another dose for that new incident.
What Happens After You Take the Pill?
Most women do not feel anything immediately. The pill works silently in the background to delay ovulation.
Some women notice mild side effects within a few hours. Nausea is the most common. Some women vomit. If you vomit within two hours of taking the pill, the dose may not be fully absorbed. Contact a pharmacist or clinician about whether you need a repeat dose.
Your next period may come earlier or later than expected. This is normal. The hormone shift can change your cycle timing by a few days.
If your period is more than seven days late, take a pregnancy test. Plan B is not 100% effective. A late period does not automatically mean pregnancy, but it is worth checking.
Does Weight Affect How Well Plan B Works?
Research indicates that body weight can influence effectiveness. Studies suggest that levonorgestrel emergency contraception may be less effective in women who weigh more than about 165 pounds.
This is not a hard cutoff. It is a trend seen in clinical research. The exact mechanism is not fully understood, but it likely relates to hormone concentration in the blood.
If you weigh more than 165 pounds, a different emergency contraceptive may be more reliable. Ulipristal acetate, sold as Ella, is another option. It works differently and appears to maintain effectiveness at higher body weights.
Your clinician can help you choose the right option. Do not assume Plan B will fail. Just be aware that alternatives exist.
How Is Plan B Different From the Abortion Pill?
Plan B is not an abortion pill. This is a common misunderstanding with serious consequences.
Plan B prevents pregnancy. It stops ovulation before fertilization can occur. It does nothing to an already implanted embryo.
The abortion pill, mifepristone, works after pregnancy has been established. It terminates an existing pregnancy. These are completely different medications with completely different mechanisms.
If you are already pregnant, Plan B will not work. It has no effect on an established pregnancy. Taking it will not harm a developing fetus, but it will not end the pregnancy either.
When Should You Use Plan B Instead of Regular Birth Control?
Plan B is for emergencies only. It is not a substitute for regular contraception.
Use it in specific situations: a condom broke, you missed multiple birth control pills, or you had unprotected sex. These are the scenarios where emergency contraception makes sense.
Regular birth control methods are far more effective over time. The pill, IUD, implant, and other methods prevent pregnancy consistently. Plan B is a backup, not a primary strategy.
Using Plan B repeatedly is not recommended. Frequent use can disrupt your cycle and cause unpredictable bleeding. It is also less effective than consistent birth control.
How Soon Can You Take a Pregnancy Test After Plan B?
Plan B delays ovulation, which delays the timeline for a possible pregnancy test. You cannot test immediately.
Most home pregnancy tests are reliable about 14 days after sex. Some sensitive tests can detect pregnancy earlier, around 10 days. Waiting the full 14 days gives the most accurate result.
Testing too early produces false negatives. The pregnancy hormone hCG takes time to build up in your system. A negative test at day five means nothing.
If you get a negative result at 14 days but still have no period, test again a week later. This catches pregnancies that were slow to produce detectable hormone levels.
Plan B Side Effects You Should Know About
Side effects are usually mild and short-lived. Most resolve within a day or two.
Common side effects include:
- Nausea
- Headache
- Dizziness
- Breast tenderness
- Fatigue
- Irregular bleeding
Nausea is the most frequently reported side effect. Some women feel sick within hours of taking the pill. Eating food before taking it may help reduce nausea.
Spotting or light bleeding between periods is common. This is not a period and not a sign of pregnancy. It is a hormonal response to the medication.
Serious side effects are rare. If you experience severe abdominal pain, unusual heavy bleeding, or signs of an allergic reaction, seek medical attention promptly.
Do You Need a Prescription for Plan B?
No prescription is required. Plan B is available over the counter in most pharmacies and drugstores.
You can buy it without showing ID or answering questions. It sits on the shelf alongside other family planning products. You do not need to speak to a pharmacist unless you have questions.
The cost varies by location and brand. Generic versions are usually cheaper than name-brand Plan B One-Step. Check prices at different stores if cost is a concern.
Some clinics and health departments offer emergency contraception at reduced cost or free. Planned Parenthood and similar organizations can help if cost is a barrier.
Does the Timing of Your Cycle Matter?
Yes, your cycle timing affects how well Plan B works. The pill only delays ovulation. If you have already ovulated, it cannot help.
Ovulation typically happens around day 14 of a 28-day cycle. But cycles vary. Some women ovulate earlier or later. Tracking your cycle helps you understand your risk.
Sperm can survive inside the body for up to five days. This means unprotected sex before ovulation can still result in pregnancy. Plan B works by pushing ovulation back far enough that the sperm die before an egg is released.
If you are past ovulation when you take Plan B, it will not work. This is not a failure of the medication. It is a timing limitation that no emergency contraceptive can overcome.
What If You Vomit After Taking Plan B?
Vomiting within two hours of taking the pill is a problem. The medication may not have been fully absorbed.
If this happens, contact a pharmacist or clinician. They can advise whether you need a second dose. Do not take a second dose on your own without guidance.
Taking anti-nausea medication before Plan B may help. Some clinicians recommend it for women who are prone to nausea. Ask a pharmacist for advice.
If you vomit after two hours, the dose is likely absorbed. No repeat dose is needed. The pill has had time to enter your system.
Frequently Asked Questions
How long do you take Plan B after unprotected sex?
You take one pill within 72 hours of unprotected sex. It works best when taken within the first 24 hours.
Can you take Plan B more than once in a cycle?
Yes, but it is not recommended as a regular method. Taking it multiple times in one cycle can disrupt your menstrual cycle and cause unpredictable bleeding.
Does Plan B work if you already ovulated?
No. Plan B delays ovulation, so it cannot work if ovulation has already occurred. This is why taking it as soon as possible matters.
How long does Plan B stay in your system?
The active hormone is mostly cleared from your body within a few days. Its effects on your cycle may last longer, which is why your next period may be irregular.
