Plan B One-Step Side Effects: What to Expect and When to Call Your Doctor

You've taken Plan B One-Step. Now you're wondering: is what I'm feeling normal? Am I having side effects? When should I actually be worried?

Plan B One-Step is a high-dose progestin (levonorgestrel 1.5 mg) taken in a single dose. Most people tolerate it well, but it does cause side effects in many users — particularly in the days immediately following. Here's what the clinical data says and what to expect.

Common Side Effects of Plan B One-Step (Reported in Clinical Trials)

The FDA clinical trial data for Plan B One-Step reported the following adverse reactions (those occurring in 10% or more of users):

Heavier menstrual bleeding: 31% of users — the most common side effect; your next period may be heavier than usual

Nausea: 14% — taking Plan B with food may help reduce nausea

Lower abdominal pain / cramping: 13% — mild cramping is normal in the days after taking Plan B

Fatigue: 13% — tiredness or fatigue is common in the first day or two

Headache: 10% — typically mild; can be managed with OTC pain relievers

Dizziness: 10% — usually mild and short-lived

Other side effects reported include vomiting, breast tenderness, and spotting or irregular bleeding between taking Plan B and your next period.

What Happens to Your Period After Taking Plan B?

One of the most common concerns after taking Plan B One-Step is what happens to your menstrual cycle. Here's what's normal:

Your period may come early or late. This is the most common menstrual change. EC pills like Plan B alter the hormonal cycle and often shift when your next period arrives.

Spotting may occur. Light bleeding or spotting between taking Plan B and your next period is normal.

Your period may be heavier or lighter than usual.

Your cycle should return to normal the following month. Taking Plan B frequently can cause irregular periods over time, which is another reason EC is not intended for routine use.

When Should You Take a Pregnancy Test After Plan B?

Take a pregnancy test if your period doesn't arrive within 3 weeks (21 days) of taking Plan B One-Step. If your period is more than a week late, this is a signal to test. A home pregnancy test will be reliable at this point.

When to Call Your Doctor or Seek Emergency Care

Most Plan B One-Step side effects are mild and resolve on their own within a few days. However, some symptoms require medical attention:

Severe lower abdominal pain: Especially on one side — this may be a sign of an ectopic pregnancy (a pregnancy outside the uterus), which is a medical emergency. Seek emergency care immediately.

Heavy, unusual bleeding: If you're soaking through more than one pad per hour for several hours, contact your healthcare provider.

Vomiting within 2 hours of taking Plan B: If you vomit within 2 hours of taking Plan B, the medication may not have been fully absorbed. Call a healthcare provider to find out if you should repeat the dose.

Symptoms of allergic reaction: Rash, hives, difficulty breathing, or swelling — rare but requires immediate medical attention.

What About Long-Term Side Effects?

Plan B One-Step has been studied extensively and is not associated with long-term side effects for most users. Key facts:

Taking Plan B does not affect your future fertility — it does not make it harder to get pregnant later

Levonorgestrel has not been shown to increase the risk of cancer with short-term use

If Plan B fails and pregnancy occurs, studies have not shown significant increase in birth defect risk

Using EC frequently can cause your periods to become irregular — this is why it's meant for emergencies, not routine birth control

Is Plan B Safe for Everyone?

Plan B One-Step is considered safe for most people. It should not be used if you are already pregnant (it won't work and isn't indicated), and people with known allergy to levonorgestrel should avoid it. It is not approved for use as a regular contraceptive — talk with your healthcare provider about ongoing birth control options.

For information about which medications can make Plan B less effective, see our article on Plan B One-Step drug interactions: what to avoid and what to tell your doctor.