Plan B works best when you let it do its job without interference. The biggest things to avoid are vomiting within the first two hours, having unprotected sex again before your next period, and taking certain medications or supplements that reduce its effectiveness. Beyond that, most normal activities are fine, but a few precautions can help you get the most protection and feel better while side effects run their course.
Avoid Vomiting in the First Two Hours
If you throw up within two hours of taking Plan B, your body may not have absorbed enough of the medication for it to work. In that case, you likely need to take another dose as soon as possible. Talk to a pharmacist to confirm whether a second pill is necessary.
Since nausea is one of the most common side effects of Plan B, this creates a frustrating catch-22. Eating a small, bland meal before or right after taking the pill can help settle your stomach. Avoid greasy, spicy, or heavy foods in the hours after you take it. If you know you’re prone to nausea, an over-the-counter anti-nausea medication taken 30 minutes beforehand can help keep the pill down.
Don’t Have Unprotected Sex Afterward
Plan B is not ongoing birth control. It addresses the single act of unprotected sex that already happened, but it does not protect you from pregnancy if you have sex again afterward. Use a barrier method like a condom for any sexual activity between now and your next period. This is one of the most common misunderstandings about emergency contraception, and skipping this step is one of the easiest ways it “fails.”
Medications and Supplements That Reduce Effectiveness
Certain drugs speed up how quickly your liver breaks down Plan B’s active ingredient, leaving less of it in your bloodstream to prevent pregnancy. If you’re currently taking any of the following, Plan B may not work as well:
- Epilepsy medications such as carbamazepine, phenytoin, and barbiturates
- Tuberculosis drugs like rifampicin and rifabutin
- HIV medications including ritonavir and efavirenz
- Antifungal medications like griseofulvin
- St. John’s Wort, a common herbal supplement used for mood support
The effect of these drugs doesn’t disappear overnight. Your liver enzyme levels can stay elevated for up to four weeks after you stop taking them. So even if you recently stopped one of these medications, Plan B’s effectiveness could still be reduced. If any of these apply to you, a copper IUD inserted as emergency contraception is a more reliable alternative, and a pharmacist or doctor can walk you through that option.
Alcohol Won’t Block It, but May Make You Feel Worse
Drinking alcohol after taking Plan B does not reduce the pill’s effectiveness. There is no known interaction between alcohol and levonorgestrel, and neither the prescribing information nor the National Institute on Alcohol Abuse and Alcoholism flags a concern.
That said, Plan B commonly causes tiredness, nausea, headaches, dizziness, and vomiting. Alcohol causes the same symptoms. Combining the two can make for a genuinely miserable few hours. If you’re already feeling queasy or wiped out, holding off on drinking for the rest of the day is a practical move, not a medical requirement.
Body Weight and Effectiveness
This isn’t something to “avoid” exactly, but it’s important to know: if you weigh more than 165 pounds, Plan B does not work as well. The hormone dose becomes less effective at higher body weights. This doesn’t mean it’s worthless, but the protection drops significantly. If this applies to you, a copper IUD or a different type of emergency contraceptive pill (containing ulipristal acetate, sold as ella) may be a better choice. A pharmacist can help you decide quickly.
Don’t Restart Hormonal Birth Control Too Soon (If You Took ella)
This only applies if you took ella (ulipristal acetate) instead of Plan B, but it’s worth knowing since the two are often discussed together. If you took ella and want to start or restart birth control pills, patches, or vaginal rings, wait until the sixth day after taking it. Starting a hormonal method containing progestin earlier than that can actually make both the emergency contraception and the birth control less effective. Use condoms during the gap, and for seven more days after you start the hormonal method.
If you took Plan B specifically (levonorgestrel), you can start or restart your regular hormonal birth control right away. Just use a backup method like condoms for the first seven days.
When to Take a Pregnancy Test
Plan B is not 100% effective, so knowing when to test matters. If your period is late, take a pregnancy test. If you’re not sure when your period is due, wait at least 21 days after the unprotected sex before testing. Testing earlier than that can give a false negative because pregnancy hormone levels may not be high enough to detect yet.
Severe Belly Pain Is a Red Flag
Most side effects from Plan B are mild: nausea, fatigue, headache, spotting, and breast tenderness. These typically resolve within a day or two. But severe abdominal pain after taking emergency contraception can be a sign of an ectopic pregnancy, where a fertilized egg implants outside the uterus. This is a medical emergency. If you experience intense, sharp pain in your lower abdomen, get medical help immediately rather than waiting it out.