Plan B can make your period up to a week late, and in some cases slightly longer. A delay of about seven days is the general range, though the exact shift depends on where you were in your menstrual cycle when you took it. Your period could also arrive early or on time, so a delay isn’t guaranteed.
Why Timing in Your Cycle Matters
Plan B delivers a large dose of a synthetic hormone similar to progesterone, and its effect on your cycle depends on when you take it relative to ovulation. If you take it early in your cycle, before ovulation, it tends to shorten your cycle. That means your period may actually come a few days sooner than expected. You may also notice spotting or light bleeding between periods, which is a direct result of the hormonal disruption in the first half of the cycle.
If you take Plan B after ovulation, in the second half of your cycle, the opposite happens. The extra hormone extends the luteal phase (the stretch between ovulation and your period), which pushes your period back. This is the scenario most people are worried about when they search for how late their period might be. A delay of up to a week is typical in this case, though some people experience slightly longer shifts.
What Your Period May Look Like
Beyond timing, Plan B can change how your period feels. Some people experience lighter or heavier bleeding than usual. Spotting before your period starts is also common, especially if you took Plan B earlier in your cycle. These changes are temporary and caused by the same hormonal disruption that shifts your timing. Your flow and duration should return to normal within a cycle or two.
If you notice very light spotting around a week after unprotected sex but no full period, that’s worth paying attention to. Light spotting can be a side effect of Plan B, but it can also be an early sign of implantation. On its own, spotting doesn’t confirm or rule out anything, so the only reliable next step is a pregnancy test at the right time.
When to Take a Pregnancy Test
If your period is more than one week late after taking Plan B, a pregnancy test is a reasonable next step. Home pregnancy tests are most accurate when taken after a missed period, but if your cycle is irregular and you’re unsure when your period is due, the NHS recommends testing at least 21 days after the unprotected sex in question. Testing earlier than that can produce a false negative because the hormone pregnancy tests detect may not be at measurable levels yet.
It’s worth knowing that many Plan B side effects overlap with early pregnancy symptoms: nausea, fatigue, dizziness, breast tenderness, and light spotting can all show up in either situation. These symptoms alone can’t tell you whether Plan B worked or not. A positive pregnancy test is the only reliable indicator that emergency contraception didn’t prevent pregnancy.
Will Your Next Cycles Be Affected?
For most people, only the first period after Plan B is noticeably different. The hormonal disruption is a one-time event, and your body typically resets within one to two cycles. If your periods remain irregular for several months afterward, that’s more likely related to something else, like stress, weight changes, or an underlying condition, rather than a lingering effect of Plan B.
The key takeaway: a period that’s a few days to a week late after Plan B is normal and expected, especially if you took it in the second half of your cycle. Anything beyond a week warrants a pregnancy test for peace of mind.