Plan B can delay your period by up to one week. Most people who take it will notice their next period arrives a few days later than expected, though some experience it earlier than usual or right on time. The direction and degree of the shift depend largely on where you are in your menstrual cycle when you take it.
What a Typical Delay Looks Like
A delay of a few days to one week is the most commonly reported change. Your period might come just two or three days late, or it could push close to a full seven days past your expected date. Both are normal responses to the high dose of synthetic progesterone in Plan B.
Some people don’t experience a delay at all. Instead, their period arrives early or on schedule but with a heavier or lighter flow than usual. You might also notice spotting in the days right after taking the pill, which is not the same as your period. That breakthrough bleeding is a separate hormonal response and doesn’t reset your cycle clock.
Why Timing in Your Cycle Matters
Plan B works primarily by delaying ovulation. It delivers a large, single dose of a hormone similar to progesterone, which temporarily halts the hormonal chain reaction that triggers egg release. Because of this mechanism, when you take it relative to ovulation heavily influences what happens next.
Some research suggests that taking Plan B earlier in your cycle, well before ovulation, may actually cause your next period to arrive sooner than expected. The logic is straightforward: if ovulation is pushed back only slightly or your body resets quickly, the hormonal surge can speed up the shedding of your uterine lining.
Taking it later in your cycle, closer to or after ovulation, is more likely to cause a delay. At that point, the extra progesterone can extend the luteal phase (the stretch between ovulation and your period), pushing your period back by several days. Research also shows that taking Plan B both before ovulation and later in the cycle can make the period itself last longer than usual once it does arrive.
Other Cycle Changes to Expect
A delayed period is the most talked-about side effect, but it’s not the only way Plan B can disrupt your cycle. Common changes include:
- Heavier or lighter bleeding than your typical period
- Spotting between periods in the days or weeks after taking the pill
- Cramping that may feel different from your usual menstrual cramps
- A longer-lasting period once bleeding does start
These irregularities usually resolve within one to two cycles. Your body is adjusting to a sudden, temporary spike in synthetic hormones, and it takes time for your natural rhythm to recalibrate. If spotting continues for more than a week, that’s worth a medical conversation.
When a Late Period Might Mean Something Else
Plan B is effective but not foolproof. If your period is more than one week late after taking it, the delay may not be a side effect. It could signal that the pill didn’t work and a pregnancy has occurred. At that point, a home pregnancy test is reliable. If you’re unsure exactly when your period was due, the general guideline is to test at least 21 days after unprotected sex for a dependable result.
Other symptoms worth paying attention to include severe lower abdominal pain, which in rare cases could indicate an ectopic pregnancy, and bleeding that is dramatically heavier than any period you’ve had before. A delay alone isn’t cause for alarm, but a delay plus unusual symptoms deserves medical attention.
Does Body Weight Affect the Delay?
There’s an ongoing debate about whether Plan B is less effective for people at higher body weights, and effectiveness and cycle disruption are linked. If the pill doesn’t suppress ovulation as strongly, it may also cause less of a period shift.
Health Canada has noted that Plan B may be less effective in people over 165 pounds and potentially ineffective over 176 pounds. However, the European Medicines Agency reviewed the same data and concluded the evidence was too limited to draw firm conclusions, advising people to continue using it regardless of weight. Canadian obstetric guidelines similarly recommend that people with a BMI over 30 should not be discouraged from using Plan B if other options aren’t accessible.
An alternative emergency contraceptive pill containing a different active ingredient (sold as ella) appears to maintain better efficacy at higher body weights, with an unintended pregnancy rate of about 2.5% compared to roughly 6% for Plan B in people with a BMI over 30. If weight is a concern and you’re within the appropriate time window, this may be a more reliable option, and its hormonal impact on your cycle would differ accordingly.
What to Do While You Wait
The hardest part for most people is the uncertainty. You took Plan B, your period is late, and you’re wondering whether it’s the pill or a pregnancy. A few practical steps can help you through that window. First, mark the date you took Plan B and your expected period date so you can accurately track how many days late you actually are versus how late it feels. Second, avoid taking a pregnancy test too early. Testing before your period is even a full week late can produce a false negative simply because hormone levels aren’t high enough to detect yet. Third, keep in mind that stress itself can delay a period, and the anxiety of waiting for a late period after emergency contraception creates a frustrating feedback loop.
If your period arrives within seven days of its expected date, even if the flow or duration feels a little off, that falls within the normal range of Plan B side effects. Your next cycle after that should return to its usual pattern.