There is no strict limit on how many times you can take Plan B. The American College of Obstetricians and Gynecologists states that it can be used more than once, even within the same menstrual cycle. That said, it’s less effective and harder on your body than regular birth control, so it works best as a backup rather than a primary method.
No Maximum Number, but Not Ideal for Routine Use
Plan B contains a large dose of a synthetic hormone (the same type found in many birth control pills, just at a much higher concentration). No medical organization sets a hard cap on how many times you can take it in a month or year. Long-term safety data on very frequent use is limited, but there’s no evidence that repeated use causes lasting harm.
The FDA labels Plan B as “not for routine contraceptive use,” which sometimes gets misread as a safety warning. The real reason is practical: it simply doesn’t work as well as regular contraception. Plan B is up to 94% effective when taken within 24 hours of unprotected sex. Daily birth control pills, used consistently, are more than 99% effective. If you find yourself reaching for Plan B regularly, switching to a daily pill, patch, ring, implant, or IUD will protect you far more reliably.
What Repeated Use Does to Your Cycle
The most noticeable consequence of taking Plan B frequently is menstrual disruption. A single dose can shift your next period by a week or more in either direction and change how heavy or light it is. These effects tend to stack. If you’ve used Plan B more than once in a single cycle, you’re more likely to experience irregular bleeding, spotting between periods, or a period that lasts longer than usual.
These changes are temporary. Your cycle typically returns to normal the following month. But if you’re taking it often enough that your periods are consistently unpredictable, that unpredictability itself becomes a problem, since it makes it harder to tell whether a late period is a side effect or a sign of pregnancy.
Side Effects With Each Dose
Every time you take Plan B, you may experience nausea, fatigue, headaches, breast tenderness, or lower abdominal cramping. These are short-lived, usually resolving within a day or two. There’s no evidence that the side effects become more severe with repeated doses, but you do go through them each time.
It Won’t Affect Your Future Fertility
A 2022 review in the journal Contraception found no indication that Plan B impairs future fertility, even with repeated use. Extensive research on the same hormone in daily contraceptive pills and implants consistently shows that conception rates return to normal after stopping, regardless of how long someone used the method. The review concluded that repeated emergency contraception use is unlikely to affect the ability to get pregnant later.
Weight Can Reduce Effectiveness
If you weigh more than about 176 pounds or have a BMI of 30 or higher, Plan B is significantly less reliable. Research from Oregon Health & Science University found that people with a BMI of 30 experienced Plan B failure four times as often as those with a BMI under 25. Blood levels of the active hormone were about 50% lower in higher-BMI individuals, meaning the dose may never reach the threshold needed to block ovulation. Doubling the dose does not fix this problem. If this applies to you, a copper IUD inserted as emergency contraception is the most effective alternative, and it also doubles as long-term birth control afterward.
How Plan B Actually Works
Plan B works primarily by delaying or preventing ovulation. If your body hasn’t yet released an egg, the large hormone dose can pause that process long enough for sperm (which survive about five days) to die off before fertilization can happen. If ovulation has already occurred, Plan B is much less effective. This is why timing matters so much: the sooner you take it after unprotected sex, the better the odds that ovulation hasn’t happened yet.
Starting Regular Birth Control After Plan B
You don’t need to wait for your next period to begin a regular contraceptive method after taking Plan B. You can start the pill, patch, ring, implant, or get an IUD inserted right away. The one requirement is using a backup method (like condoms) for the first seven days after starting your new contraception, since it takes about a week for most hormonal methods to become fully effective.
A copper IUD is the exception. If inserted after taking Plan B, it provides immediate protection with no backup method needed, and it remains effective for up to 10 or 12 years depending on the type.
If you used a different emergency contraceptive called ella (ulipristal acetate) instead of Plan B, the timeline is different. You need to wait six days before starting any hormonal birth control, because the two medications can interfere with each other’s effectiveness.