How Long Do Plan B Side Effects Last?

Quick Answer

Most Plan B (levonorgestrel) side effects resolve within 24–48 hours. Nausea, the most frequently reported side effect, usually passes within 24 hours. Some effects on your menstrual cycle can persist into the following month.

Common Side Effects and Timeline

Side Effect	How Common	How Long It Lasts
Nausea	~23% of users	12–24 hours
Headache	~17% of users	12–48 hours
Fatigue	~17% of users	24–48 hours
Lower abdominal pain	~18% of users	24–48 hours
Dizziness	~11% of users	A few hours
Breast tenderness	~11% of users	1–3 days
Irregular bleeding/spotting	~16% of users	Up to a few days

These percentages come from the manufacturer’s clinical data. Most people experience only one or two of these side effects, and many people experience none at all.

Effect on Your Next Period

Plan B can affect the timing of your next menstrual period. Your period may arrive up to a week early or a week late. If your period is more than 7 days late, consider taking a pregnancy test. The flow may also be heavier or lighter than usual for one cycle, but this should normalize in the following month.

How Plan B Works

Plan B contains 1.5 mg of levonorgestrel, a synthetic progestin hormone. It works primarily by delaying or preventing ovulation. If ovulation has already occurred, it may prevent fertilization or implantation. It does not terminate an existing pregnancy.

Efficacy Window

Plan B is most effective when taken as soon as possible after unprotected sex:

• Within 24 hours: ~95% effective
• 24–48 hours: ~85% effective
• 48–72 hours: ~58% effective

The medication can be taken up to 72 hours (3 days) after unprotected intercourse, but effectiveness decreases significantly with each passing day. Ella (ulipristal acetate) is an alternative that remains effective up to 120 hours (5 days).

Who Should Not Take Plan B

Plan B is generally considered safe for most people, but you should consult a healthcare provider if you:

• Are allergic to levonorgestrel or any inactive ingredients
• Weigh over 165 pounds (efficacy may be reduced; ella or a copper IUD may be better options)
• Are already pregnant (the medication will not work, though it is not known to harm an existing pregnancy)
• Are taking certain medications that may interact, such as barbiturates, bosentan, carbamazepine, or St. John’s Wort

When to Seek Medical Attention

Contact a healthcare provider if you experience:

• Severe abdominal pain (could indicate ectopic pregnancy)
• Vomiting within 2 hours of taking the pill (you may need another dose)
• No period within 3 weeks of taking Plan B
• Side effects that persist beyond 48 hours or worsen

Tips for Managing Side Effects

• Take with food to reduce nausea
• Anti-nausea medication (like meclizine) taken 30 minutes beforehand can help
• Stay hydrated and rest if you feel fatigued
• Over-the-counter pain relievers can help with headaches and cramps