Plan B is commonly used after unprotected sex, but it's not foolproof. Its effectiveness depends on several key factors — including timing, body weight, and whether you've already ovulated. Understanding how Plan B works and when it’s most effective can help you make informed choices.

What Is Plan B and How Does It Work?

Plan B, also called the “morning after pill,” is an over-the-counter emergency contraceptive. It contains levonorgestrel, a synthetic hormone (progestin) that works primarily by preventing or delaying ovulation—so that sperm doesn’t have an egg to fertilize.

Important: Plan B is not the abortion pill. It does not end an existing pregnancy.

Can Plan B Fail?

Yes. Plan B doesn’t always work. Here’s why:

1. You’ve Already Ovulated

Plan B won’t stop a pregnancy if ovulation has already occurred. That’s why tracking your menstrual cycle is so important—especially if you rely on timing-based contraception.

2. Body Weight May Affect Absorption

Studies show Plan B may be less effective in women over 175 lbs. This doesn’t mean it won’t work at all—but effectiveness may drop due to lower absorption.

3. Medications

Medications, such as St John’s Wort, or seizure medications that can interact with Plan B, making it less effective.

Learn more about the factors that affect Plan B’s performance.

What Are the Common Side Effects of Plan B?

Most side effects are mild and go away within a few days. You might experience:

• Nausea or cramping
• Fatigue or headache
• Breast tenderness
• Irregular bleeding
• Changes in your next period

What Should I Expect After Taking It?

Your period might come earlier or later than usual. If it’s more than 7 days late, consider taking a pregnancy test.

Unusual symptoms like severe pain or heavy bleeding could be signs of something more serious—such as an ectopic pregnancy. Contact your healthcare provider if anything feels off.

Final Thoughts: Is Plan B Reliable?

Plan B is an emergency option—but it’s not 100% effective. Here’s what matters most:

• Time is a factor
• Know your ovulation window
• Understand how your weight might affect it
• Always consult a medical provider if you’re unsure

Need Someone to Talk To?

If you have questions about Plan B, your options, or your next steps—we’re here for you. Our licensed medical team offers free, confidential consultations in a judgment-free environment.

Schedule a free appointment today

FAQs: Plan B & Emergency Contraception

Q: Is Plan B the same as the abortion pill?
A: No. Plan B prevents pregnancy before it starts. It does not end a pregnancy that’s already begun.

Q: Can I take Plan B more than once?
A: Yes, but it’s not recommended as a regular form of birth control. Talk to your provider about better long-term options.

Q: Does Plan B work during ovulation?
A: It’s less effective or may not work at all if you’ve already ovulated.

Q: Is there a weight limit for Plan B?
A: There’s no strict cutoff, but effectiveness may decrease if you weigh over 175 lbs.