Plan B can delay your period by up to one week, according to the Mayo Clinic. Some people get their period right on time or even a few days early. The direction and degree of the shift depends on where you are in your menstrual cycle when you take the pill.
Why Plan B Shifts Your Cycle
Plan B contains a synthetic hormone called levonorgestrel that works by stopping or delaying ovulation. The FDA has confirmed that it prevents pregnancy by inhibiting the release of an egg and disrupting the midcycle hormonal changes that trigger ovulation. It does not affect fertilization or implantation after ovulation has already occurred.
Because ovulation is what sets the clock for the second half of your cycle, delaying it pushes everything back. If you take Plan B early in your cycle, before ovulation, your period is more likely to arrive late. If you take it later in your cycle, closer to when your period was already due, you may notice little change in timing or your period could even come slightly early.
What Your Next Period Might Look Like
Beyond timing, the period itself can look different. You may notice a lighter or heavier flow than usual. Some people experience just a few days of light spotting instead of a full period, while others deal with a stronger flow that calls for extra protection, like a tampon and a pad together. These changes are temporary and caused by the hormonal surge from the pill.
You might also notice spotting a few days after taking Plan B, well before your actual period is due. This breakthrough bleeding is not a period. It typically appears as a few spots of pink, red, or brown blood on your underwear or toilet paper, and it can last a day or two. Unlike a real period, it’s usually too light to need anything more than a panty liner, and it doesn’t come with the heavy cramps you might normally feel. It can, however, arrive alongside other Plan B side effects like nausea, headache, fatigue, breast tenderness, or dizziness.
When to Take a Pregnancy Test
If your period is more than a week late, that alone isn’t necessarily a sign of pregnancy. But if it hasn’t arrived within three weeks of taking Plan B, take a pregnancy test. Most home pregnancy tests are reliable starting on the first day of a missed period. If your cycle is irregular and you’re not sure when your period is due, the NHS recommends testing at least 21 days after the unprotected sex that prompted you to take Plan B.
Weight Can Affect How Well Plan B Works
One factor that many people don’t know about is that Plan B becomes significantly less effective at higher body weights. Research published in a clinical trial found that the standard 1.5 mg dose appeared to hit a ceiling of effectiveness at about 155 pounds (70 kg), with essentially no measurable effectiveness for people weighing 176 pounds (80 kg) or more. People with a BMI of 30 or above achieved peak blood levels of the drug roughly 50% lower than those with a BMI under 25.
This matters for the period question because if Plan B didn’t successfully delay ovulation due to reduced absorption, the hormonal disruption to your cycle may be minimal, and a late period could more plausibly signal pregnancy. UK clinical guidelines suggest that people at higher weights consider a double dose (3.0 mg) or a different type of emergency contraception that remains effective across a wider weight range. A copper IUD is the most effective emergency contraception option regardless of weight and can be placed up to five days after unprotected sex.
Effects on Future Cycles
Plan B is a one-time hormonal dose, not an ongoing medication. It disrupts the cycle you’re currently in, but it does not alter the regularity or length of your cycles going forward. By your second period after taking it, your cycle should return to its normal pattern. If it doesn’t, that’s worth discussing with a healthcare provider, as something else may be affecting your cycle.