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Answer:The emergency contraceptive Plan B is made from levonorgestrel—a common ingredient of many birth control pills. Some of the side effects are similar to those of certain birth control pills. Common Plan B side effects are nausea, abdominal pain, tiredness, headache, dizziness, and breast tenderness.

Note: If you vomit within an hour of taking either of the Plan B pills (the first pill should be taken within 72 hours of unprotected sex and the second pill should be taken 12 hours after the first), contact your healthcare provider for directions on what to do.

Some women may experience changes in their menstrual cycle after taking Plan B—spotting or bleeding before the next expected cycle, heavier or lighter bleeding—or start their period earlier or later than usual. However, most women get their period at the expected time or within a week of the expected time.

If your period is late more than a week beyond its expected date, you may be pregnant and should contact your healthcare provider.

Remember that Plan B is not for routine use. In addition, it does not protect against sexually transmitted diseases, including HIV/AIDS.

Get more information about Plan B.

Learn more about safe sex and birth control in the Walgreens.com Health Library.

View the Walgreens.com Ask a Pharmacist Archives for a list of all commonly asked questions.

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