What Happens to Your Body When You Take Plan B?

Plan B delivers a large single dose of a synthetic hormone called levonorgestrel, and its primary job is to delay or block ovulation so that sperm and egg never meet. That hormonal surge is far higher than what a daily birth control pill provides, so your body notices it. Most people experience only mild, short-lived effects, but the specifics depend on where you are in your menstrual cycle when you take the pill and on factors like body weight.

How the Pill Works Inside Your Body

Levonorgestrel is a synthetic version of progesterone, a hormone your body already produces. When you swallow the 1.5 mg tablet, your gut absorbs it quickly and blood levels spike within about two hours, then taper off over the next day or two.

The main thing levonorgestrel does at that dose is interfere with the hormonal chain of events that triggers ovulation. Your body normally releases a surge of luteinizing hormone (LH) right before an egg is released from the ovary. Plan B can suppress or delay that LH surge, keeping the egg locked in the follicle. If the egg never comes out, fertilization cannot happen. This works best when the pill is taken before the LH surge has begun; once the surge is already underway, the drug’s ability to stop ovulation drops sharply.¹

A common concern is whether Plan B prevents a fertilized egg from implanting in the uterus. A review of the research found that nine out of ten studies showed no difference in the uterine lining’s receptivity after levonorgestrel exposure compared to controls, and that taking the pill after ovulation resulted in conception rates similar to placebo.² In practical terms, Plan B works by preventing fertilization, not by disrupting an already-established pregnancy.

The First Few Hours and Days

Because you are taking a concentrated hormone dose all at once, side effects tend to appear within the first few hours. The most commonly reported ones are nausea, fatigue, and vomiting.³ The nausea is usually mild. If you do vomit within three hours of swallowing the pill, you may need to take another dose because your body might not have absorbed enough of the drug.⁴

Headache, breast tenderness, dizziness, and lower abdominal cramping are also reported, though less frequently. These symptoms are similar to what some people feel before a period, which makes sense because the same class of hormone is involved. Most of these effects resolve within a day or two as the drug clears your system. The terminal half-life of levonorgestrel at this dose averages roughly 24 to 46 hours, meaning the drug is largely gone from your bloodstream within a few days.⁵

What Happens to Your Next Period

One of the most noticeable aftereffects is a change in your menstrual cycle. In a study of 232 participants, researchers observed changes in cycle length, period length, and the appearance of menstrual bleeding compared to each person’s baseline. About 15% experienced spotting between periods. The specific changes depended on when in the cycle the pill was taken: women who took it well before their expected ovulation tended to see their period come earlier, while those who took it well after ovulation sometimes experienced a delayed period.⁶

The reassuring finding is that the majority of these changes disappeared by the following cycle. Your body’s hormonal rhythms typically reset themselves within one to two months. A period that arrives a week early or a few days late after Plan B is not a sign that something is wrong; it is a predictable consequence of the hormonal disruption the pill caused. If your period is more than a week late, though, a pregnancy test is a reasonable next step.

Does Body Weight Affect How Plan B Works?

This has been one of the more debated questions around emergency contraception. A systematic review found that women with a BMI of 30 or above faced roughly a fourfold increase in the risk of pregnancy after using levonorgestrel compared to women in lower BMI categories. The same analysis suggested that at body weights above about 80 kg (roughly 176 pounds), the pregnancy rate rose above 6%, which is close to the estimated pregnancy probability without any contraception at all. At weights under about 75 kg, the pregnancy rate stayed below 2%.⁷

The pharmacokinetic reason is straightforward: the same fixed dose of drug gets distributed across more body mass, which means lower peak blood concentrations where they count. A study comparing drug levels across weight groups confirmed that obese and extremely obese participants had lower and more variable levonorgestrel peaks than normal-weight participants, even though the absorption was still rapid in all groups.⁵

A separate pooled analysis of World Health Organization data did find a large effect for obesity, but the result was heavily influenced by one study site. When that site was excluded, obesity was no longer a statistically significant risk factor, though the remaining studies had too few obese participants to rule out a meaningful decrease in efficacy.⁸ The honest summary is that Plan B likely works less well at higher body weights, but the data are messy enough that no one can give you a clean cutoff. If you weigh over about 165 pounds and need emergency contraception, it is worth knowing that a copper IUD or ulipristal acetate (sold as ella) may be more effective alternatives.

The Timing Window and Why Sooner Matters

Plan B is officially approved for use within 72 hours of unprotected intercourse, but the drug works best when taken as soon as possible. A study comparing outcomes before and after the 72-hour mark found that the pregnancy rate was 0.8% when the pill was taken within 72 hours versus 1.8% when taken between 72 and 120 hours. Effectiveness ranged from about 87% to 90% in the earlier window and dropped to roughly 72% to 87% in the later one.⁹ The pill still offered significant protection at four and five days, but the trend is clear: every hour that passes lowers your odds of it working.

The reason ties directly back to ovulation. If you take the pill early enough in your cycle, you catch the LH surge before it starts and ovulation is reliably blocked. If you take it later, the surge may already be in progress and the drug has less to work with. And if ovulation has already happened, Plan B has essentially no remaining mechanism to prevent pregnancy, since it does not block fertilization after the fact or prevent a fertilized egg from implanting.²

Is It Safe to Take Plan B More Than Once?

There is a persistent belief that Plan B is only meant to be used once or twice in a lifetime, or that repeated use causes lasting harm. The evidence does not support this. A Cochrane review looking at repeated use of pericoital levonorgestrel found the method to be reasonably effective and safe across multiple uses. Menstrual irregularities were the most commonly reported side effect, but the studies found no consistent link between how often the pill was taken and how much bleeding changed. Non-menstrual side effects were described as mild, and most participants said they liked the on-demand method despite the irregular bleeding.¹⁰

That said, Plan B is not a great long-term contraceptive strategy for practical reasons. Its failure rate when used repeatedly is higher than that of regular birth control methods, and the menstrual disruptions can make cycle tracking difficult. Clinicians generally recommend transitioning to a more reliable routine method if you find yourself needing emergency contraception frequently. But the concern that using it repeatedly will somehow damage your reproductive system or impair future fertility is not backed by evidence. The same review of implantation studies cited earlier found no evidence that levonorgestrel exposure affects fetal development, miscarriage rates, stillbirth, or subsequent menstrual cycles.²

What About Teenagers?

Adolescents metabolize levonorgestrel the same way adults do, and the side-effect profile appears to be the same. A study specifically examining emergency contraception in adolescents found that nausea, fatigue, and vomiting occurred at similar rates regardless of age, and that the mean duration of menses was comparable before and after treatment. No serious adverse events were reported, and virtually all participants used the drug correctly.³ The physiological response to the pill is not meaningfully different for a 16-year-old versus a 30-year-old.

How Plan B Compares to Other Emergency Options

Plan B is not the only emergency contraceptive available, and comparing the options helps clarify what levonorgestrel actually does and does not do.

Ulipristal acetate (ella) works through a different mechanism. It is a selective progesterone receptor modulator, and it can delay ovulation even when taken closer to the LH surge than levonorgestrel can. Ulipristal also maintains consistent effectiveness for a full five days after unprotected intercourse, and that effectiveness does not decline over the window the way levonorgestrel’s does.¹ If you are more than 72 hours out from unprotected sex, or if you weigh more, ella has a meaningful edge. It does require a prescription in the United States, whereas Plan B is available over the counter.

The copper IUD is the most effective emergency contraceptive available. Its mechanism is distinct from hormonal methods: copper ions interfere with sperm function and also change the uterine environment to reduce the chance of implantation if fertilization has already occurred.¹¹ The copper IUD can be inserted up to five days after unprotected intercourse and has a failure rate well under 1%, regardless of body weight. The trade-off is that it requires a clinical visit and insertion procedure, so it is less accessible in a time crunch. Once placed, though, it doubles as ongoing contraception for up to a decade.

Ectopic Pregnancy and Plan B

You may have heard that Plan B increases the risk of ectopic pregnancy, where a fertilized egg implants outside the uterus, usually in a fallopian tube. The relationship here is more nuanced than the headlines suggest. A systematic review of pregnancies occurring after levonorgestrel treatment found that about 1% of those pregnancies were ectopic, which is roughly in line with the background rate of ectopic pregnancy in the general population.¹²

A separate case-control study found that when any contraceptive method fails and pregnancy occurs, the pregnancy is more likely to be ectopic compared to pregnancies in women who were not using contraception at all.¹³ This makes sense statistically: the methods that prevent pregnancy are better at preventing normal uterine pregnancies than they are at preventing ectopic ones, so when the method fails, the surviving pregnancies are skewed toward ectopic locations. Plan B does not cause ectopic pregnancies, but if you become pregnant despite taking it, the chance that the pregnancy is ectopic is somewhat elevated compared to a pregnancy conceived without any contraception. This is a reason to seek medical attention if you experience severe one-sided abdominal pain or unusual bleeding after a failed emergency contraception attempt.

Emotional Side Effects

Plan B is a hormonal intervention, and hormones can affect mood. Some people report feeling anxious, irritable, or tearful in the day or two after taking the pill. Disentangling the hormone effect from the stress of the situation that led to taking it in the first place is genuinely difficult. An early study of patients requesting emergency contraception measured anxiety, guilt, and depression before treatment and found elevated scores on all three factors. Two weeks after treatment, those scores had significantly decreased. The researchers attributed the initial distress primarily to the circumstances rather than the drug itself, and the post-treatment improvement suggested that resolving the pregnancy scare contributed more to emotional state than any pharmacological mood effect.

If you feel emotionally off for a few days after taking Plan B, that is common and temporary. Persistent mood changes lasting more than a week or two are unlikely to be caused by the drug, given how quickly levonorgestrel clears your body.

What Plan B Does Not Do

It is worth naming a few things the pill will not cause, since misinformation on this topic is widespread. Plan B does not terminate an existing pregnancy. If a fertilized egg has already implanted in the uterine wall, levonorgestrel will not dislodge it. It is not the same drug as mifepristone, which is used in medication abortions. Plan B also does not protect against sexually transmitted infections, does not affect long-term fertility, and does not accumulate in your system with repeated use. There is no evidence that it causes birth defects if taken unknowingly during early pregnancy.²

One misconception that circulates online is that Plan B “resets your hormones” or causes lasting hormonal imbalance. It does not. The drug creates a temporary spike that interferes with one specific hormonal event, then it is metabolized and excreted. Your body’s hormonal rhythms resume their normal pattern, usually by the next cycle. The disruption is real but brief, more like a speed bump than a detour.

1
PubMed. Ulipristal acetate: critical review about endometrial and ovulatory effects in emergency contraception
2
PubMed. Effect of levonorgestrel emergency contraception on implantation and fertility: A review
3
PubMed. Tolerability of levonorgestrel emergency contraception in adolescents
4
Pediatrics. Emergency Contraception
5
PubMed Central. Pharmacokinetics of the 1.5 mg levonorgestrel emergency contraceptive in women with normal, obese and extremely obese body mass index
6
PubMed Central. Menstrual bleeding patterns following levonorgestrel emergency contraception
7
PubMed Central. Safety and effectiveness data for emergency contraceptive pills among women with obesity: a systematic review
8
PubMed Central. Effect of BMI and body weight on pregnancy rates with LNG as emergency contraception: analysis of four WHO HRP studies
9
PubMed. Effectiveness of emergency contraceptive pills between 72 and 120 hours after unprotected sexual intercourse
10
PubMed Central. Repeated use of pre- and postcoital hormonal contraception for prevention of pregnancy
11
PubMed. Emergency contraception — mechanisms of action
12
PubMed Central. Ectopic Pregnancy and Emergency Contraceptive Pills: A Systematic Review
13
PLoS ONE. Contraceptive Use and the Risk of Ectopic Pregnancy: A Multi-Center Case-Control Study