Plan B Explained: What Emergency Contraception Does and Does Not Do
Plan B One-Step and similar generic products contain levonorgestrel, a form of emergency contraception. These pills can reduce the chance of pregnancy after unprotected sex or birth control failure.
Confusion about how Plan B works can prevent people from using it when it may be helpful.
What Does Plan B Do?
Plan B primarily works by delaying or preventing ovulation. If an egg is not released, sperm cannot fertilize it.
It does not end an established pregnancy. Plan B is not the same as medication abortion, and it will not cause an abortion if someone is already pregnant.
It also does not affect future fertility.
When Might Emergency Contraception Be Needed?
Emergency contraception may be considered after:
- Sex without contraception
- A condom breaking or slipping
- Missed birth control pills
- A delayed injection
- A diaphragm or cervical cap moving out of place
- Sexual assault
- Uncertainty about whether contraception was used correctly
Plan B works best when taken as soon as possible. Its effectiveness decreases with time.
Other emergency contraception options may remain effective for up to five days after unprotected sex. These include prescription ulipristal acetate and certain IUDs placed by a healthcare professional.
Myth: Plan B Works the Same at Any Time in the Cycle
Because levonorgestrel mainly works by delaying ovulation, it may not be effective if ovulation has already occurred.
Most people cannot identify the exact day of ovulation with certainty. Taking emergency contraception promptly is generally better than waiting.
Myth: Plan B Protects Against Future Sex
Plan B only addresses the recent episode of unprotected sex. It does not provide reliable contraception for sexual activity later in the cycle.
Use condoms or begin an appropriate ongoing birth control method according to medical guidance.
Myth: Plan B Protects Against Sexually Transmitted Infections
Emergency contraception does not prevent HIV or other sexually transmitted infections.
Testing, condoms, and timely medical evaluation may be needed after possible exposure. Some preventive treatments are time sensitive.
What Side Effects Can Occur?
Temporary side effects may include:
- Nausea
- Fatigue
- Headache
- Dizziness
- Breast tenderness
- Abdominal discomfort
- Spotting
- A period that arrives earlier or later than expected
If vomiting occurs soon after taking the medication, contact a pharmacist or healthcare provider because another dose may be needed.
When Should You Take a Pregnancy Test?
Take a pregnancy test if your period is more than approximately one week later than expected or if three weeks have passed since using emergency contraception and you have not had a normal period.
Seek urgent care for severe abdominal pain, fainting, or heavy bleeding.
Weight and Medication Considerations
Levonorgestrel emergency contraception may be less effective at higher body weights. Certain medications and supplements can also reduce its effectiveness.
A pharmacist or healthcare provider can help determine whether ulipristal acetate or an IUD may be a better option.
Emergency contraception is backup care, not something to feel ashamed about. Women’s Health and Menopause Center can help patients understand their options and choose an ongoing contraceptive method that fits their health needs.
