You’re staring at the box in the pharmacy aisle, or maybe you’ve already swallowed the pill and you’re watching the clock like it’s a time bomb. It’s a stressful spot to be in. Honestly, the most common question—the one that keeps people up at 2:00 AM—is basically: how long does Plan B work after you've had unprotected sex?
You’ve probably heard the "72 hours" rule. It’s plastered on every package. But biology isn't always a digital countdown.
The short answer? It works for up to three days, but its effectiveness drops the longer you wait. It’s not a magic "undo" button that works perfectly until the clock strikes 72. It’s more like a fading signal. The sooner you take it, the better the odds that it actually does what it’s supposed to do.
The Science of the "Morning After" Window
Plan B One-Step (and its generic cousins like Take Action or My Way) uses a synthetic hormone called levonorgestrel. Think of it as a massive dose of the progestin found in regular birth control pills. Its primary job is to act as a roadblock. It tells your brain to delay ovulation. If the egg never leaves the station, the sperm—which can hang around your reproductive tract for up to five days like unwanted houseguests—have nothing to fertilize.
Here is the kicker: Plan B is useless if you have already ovulated.
Once that egg is out, levonorgestrel doesn't do much. This is why the timing is so incredibly sensitive. If you’re wondering how long does Plan B work after an accident, you have to realize you’re racing against your own hormonal cycle.
✨ Don't miss: Horizon Treadmill 7.0 AT: What Most People Get Wrong
A study published in The Lancet and frequently cited by major health organizations like ACOG (American College of Obstetricians and Gynecologists) shows that levonorgestrel emergency contraception reduces the risk of pregnancy by about 95% when taken within 24 hours. If you wait until that 48-to-72-hour window, the efficacy drops to somewhere around 58% to 61%. That’s a pretty massive gap. It's the difference between a high-confidence safety net and a coin flip.
Does it work after 72 hours?
Technically, some providers say you can take it up to 120 hours (five days) later, but honestly, you’re pushing your luck. By day four or five, Plan B is significantly less effective. If you’ve passed the three-day mark, most doctors, including those at Planned Parenthood, will steer you toward Ella (ulipristal acetate) or a copper IUD, both of which stay highly effective for the full five-day window.
Weight, Metabolism, and the "Hidden" Effectiveness Gap
This is something a lot of people don't talk about, and it's kinda frustrating that it isn't more common knowledge. Your body weight might actually change how long or how well Plan B works.
Research suggests that levonorgestrel may be less effective for individuals who weigh over 165 pounds or have a BMI over 25. In some cases, it might not work at all. It’s not that the pill is "bad," it’s just that the standard 1.5mg dose might not be enough to suppress ovulation in a larger body. If you fall into this category, don't panic, but you should probably talk to a clinician about Ella or an IUD, which don't have the same weight-related efficacy drops.
Common Myths That Just Won't Die
We need to clear some things up because there is a lot of junk info out there.
🔗 Read more: How to Treat Uneven Skin Tone Without Wasting a Fortune on TikTok Trends
First off, Plan B is not an "abortion pill." It won't work if you're already pregnant. It won't hurt an existing pregnancy. It’s a preventative measure, not a corrective one. If the egg has already met the sperm and nestled into the uterine lining, Plan B will just wave at it as it passes through your system.
Secondly, taking two doses doesn't make it "stronger" or "longer-lasting." It just makes you twice as likely to feel like garbage. Nausea is the most common side effect, and doubling up usually just triggers a session over the toilet. If you vomit within two hours of taking the pill, though, that's a different story—you actually might need another dose because your body likely didn't absorb the first one.
What to Expect After You Take It
So, you took it. Now what?
Your next period is probably going to be weird. That’s the most honest way to put it. Because you just hit your system with a high dose of hormones, your cycle might be a week early or a week late. It might be heavier. It might be a spotty mess.
- Spotting: Very common. Don't mistake it for a period.
- Nausea: About 25% of people feel localized "blah" in their stomach.
- Breast tenderness: Feels a lot like PMS.
- Headaches: Usually mild, but they happen.
If your period is more than a week late, take a test. It’s the only way to know for sure. It’s simple, but people often forget that the "wait and see" game is the most stressful part of the process.
💡 You might also like: My eye keeps twitching for days: When to ignore it and when to actually worry
Real Talk on Better Alternatives
If you find yourself constantly asking how long does Plan B work after because of frequent mishaps, it might be time to look at the alternatives that actually have a better track record.
Ella is a prescription-only emergency contraceptive. It’s more effective than Plan B, especially as you get closer to that 5-day limit. Then there’s the Copper IUD (Paragard) or certain hormonal IUDs (Mirena/Liletta). If a doctor inserts one within five days of unprotected sex, it is more than 99% effective. It is the most powerful "oops" button we have in modern medicine. Plus, it stays in there and protects you for years.
Practical Next Steps for Your Health
If you are currently in the window and need to act, here is the move.
First, get to a pharmacy immediately. Don't wait until tomorrow morning. Every hour counts when you are trying to beat ovulation. If you are over 165 pounds, try to call a telehealth provider or a local clinic to ask for a prescription for Ella instead of buying Plan B over the counter.
Once you’ve taken the pill, mark the date on your calendar. You need to know when your next period is supposed to arrive so you aren't guessing later. If three weeks pass and you haven't seen a period, grab a pregnancy test. Even the cheap ones at the dollar store are highly accurate at that point.
Finally, if you’re using Plan B as your primary method of birth control, your wallet and your hormones are going to take a hit. It’s meant for emergencies. Talk to a provider about a long-term option like the pill, the patch, or an IUD so you don't have to keep racing the 72-hour clock. Use a backup method like condoms for at least seven days after taking emergency contraception, as your cycle will be unpredictable and you could potentially ovulate later than usual.