Plan B can delay your period by up to one week in most cases, though delays of two weeks or more are possible for a smaller percentage of users. About 13% of women experience a delay longer than seven days. The shift happens because the high dose of hormones in Plan B disrupts your normal cycle timing, particularly ovulation.
Why Plan B Shifts Your Cycle
Plan B works primarily by delaying or preventing ovulation. It delivers a large dose of a synthetic hormone that suppresses the natural hormonal surge your body uses to trigger egg release. When ovulation gets pushed back, everything downstream in your cycle shifts with it, including the buildup and shedding of your uterine lining. That’s why your period arrives late.
The timing of this delay depends heavily on where you are in your cycle when you take the pill. If you take Plan B early in your cycle, before your body has started gearing up for ovulation, the delay tends to be longer because ovulation gets pushed further out. If you take it later in your cycle, closer to or after ovulation has already occurred, your period may arrive on time or even a few days early. In clinical studies, some women who took the medication right before ovulation experienced delays averaging around 17 days from their expected ovulation date, while others in the same study saw no change to their cycle length at all.
What the First Period After Plan B Looks Like
Your first period after taking Plan B may not feel or look like a typical one. The flow can be heavier or lighter than usual, and the duration may be shorter or longer. Some people notice stronger cramping, while others report a surprisingly mild period. These variations are all normal responses to the hormonal disruption.
Around 30% of users experience some bleeding within seven days of taking the pill, well before their actual period is due. This breakthrough bleeding is lighter than a period, often showing up as spotting or light pink or brown discharge. It happens because the sudden hormone surge can cause a small amount of uterine lining to shed early. This is not your period. Your real period will still come later, on its own timeline.
The easiest way to tell the difference: breakthrough bleeding is typically light, lasts one to three days, and doesn’t follow the pattern of gradually getting heavier. A true period starts light, builds to a heavier flow, and tapers off over several days. By the time your actual period arrives, most of Plan B’s side effects (nausea, headaches, fatigue) have long since resolved, so the period itself usually feels relatively normal aside from potential changes in flow.
When a Late Period Means You Should Test
Plan B is about 87% effective when taken within 72 hours of unprotected sex, which means it doesn’t work every time. A late period is the most common side effect and usually not a sign of pregnancy, but you can’t tell the difference between a hormone-related delay and a missed period from pregnancy based on timing alone.
If your period is more than a week late, take a home pregnancy test. Planned Parenthood recommends testing three weeks after taking the pill for the most reliable result. That window gives your body enough time to produce detectable levels of pregnancy hormones if conception occurred. Testing earlier than three weeks can produce a false negative, so if you get a negative result before that point and your period still hasn’t arrived, test again once you hit the three-week mark.
What Affects How Long Your Delay Lasts
Several factors influence whether your period comes back in a few days or takes a couple of weeks:
- Cycle timing: Taking Plan B in the first half of your cycle, before ovulation, tends to cause longer delays. Taking it in the second half, after ovulation, usually has less impact on timing.
- Individual hormone response: In clinical research, some women’s ovulation was significantly delayed by the medication while others in the same study group saw no measurable effect on their cycle. Bodies simply respond differently to the same dose.
- Baseline cycle regularity: If your cycles are already irregular, it can be harder to gauge whether Plan B caused a delay or your period was going to be late anyway.
Your cycles should return to their normal pattern within one to two months. Plan B affects only the cycle in which you take it. It does not cause lasting changes to your fertility or long-term cycle regularity. If your periods remain irregular beyond two full cycles, something else may be going on.