The best time to take Plan B for maximum effectiveness

• The best time to take Plan B is within 24 hours of unprotected sex.
• Its effectiveness is 85% if taken two days after having unprotected sex and 61% after three days.
• You shouldn't take multiple Plan B pills unless you vomit or have more unprotected sex.

Plan B, commonly known as "the morning-after pill," can effectively prevent you from becoming pregnant, but only if you take it at the right time.

And while there's no limit to how many times you can take Plan B, that doesn't mean you should treat it like a standard birth control pill that you take regularly.

How long can you wait to take Plan B?

The sooner you take Plan B after unprotected sex, the better. "The efficacy of Plan B is the best when you take it within 12 hours, and it goes down from there," says Nichole Butler, MD, board-certified OB-GYN at Weiss Memorial Hospital's Women's Health Center.

You can take Plan B up to 72 hours after unprotected sex. However, the pill's effectiveness at preventing pregnancy dwindles the later you take it:

• Within 24 hours of sex: 95% effective
• Between 24-48 hours after sex: 85% effective
• Between 48-72 hours after sex: 61% effective

It only takes a couple of hours for Plan B to kick into effect, says Butler. A 2018 review found that peak levels of levonorgestrel (the active ingredient in Plan B) in your blood will typically be around 1.7 hours after taking the pill.

Note: If you weigh 155 pounds or more, you should opt for another kind of emergency contraception pill called Ella. Ella can be used up to 120 hours after unprotected sex, though it's more effective the sooner you take it.

When you may need to take multiple doses of Plan B

You only need one dose for each episode of unprotected sex. "Taking more than one dose of Plan B after one episode of unprotected sex is not going to make a difference in its effectiveness," says Aparna Sridhar, MD, an OB-GYN at UCLA Health.

But there are some exceptions to this rule, like:

• Vomiting: If you become nauseous and vomit within two hours of taking Plan B, this is a case where you may need to take another dose, since the pill didn't have the chance to really enter your system and do its job. In this situation, call your doctor.
• More unprotected sex: Let's say you had unprotected sex again, a day or two after taking Plan B. In that case, you should consider taking another dose, since Plan B is really only meant to prevent pregnancy for one instance of unprotected sex.

That said, Sridhar also advises against taking Plan B multiple times a week. While it's not technically dangerous to your health, you may experience short-term side effects like headache, fatigue, nausea, and temporary changes in your menstrual cycle.

Moreover, if you find yourself in the situation where you're taking Plan B multiple times a week, you should strongly consider going on birth control regularly for a couple of reasons:

• Using birth control properly will be more effective at preventing pregnancy than Plan B.
• Emergency contraception, at $11 to $45, is usually more expensive than other forms of birth control, like a box of condoms for $2 to $6 or prescription birth control that most health insurance plans will typically cover.

When is it too late to take Plan B?

If you wait multiple days to take Plan B, chances of preventing pregnancy aren't great.

A 2011 meta-analysis found that taking emergency contraception on the fifth day (120 hours) after unprotected sex makes you five times more likely to get pregnant than if you took it within 24 hours.

Waiting five days increases the odds that you ovulated in that time period, and therefore, there's a chance that a sperm may have already been able to fertilize a released egg, Butler says.

Furthermore, sperm are able to live inside your body for up to a maximum of five days. The bottom line is, waiting five days after unprotected sex to take Plan B is leaving a lot up to chance. "At 120 hours, you're really crossing your fingers," says Butler.

Additionally, taking Plan B will not harm or terminate an existing pregnancy, Butler says, and it does not act as an abortion pill. "If there is an embryo implanted in the lining [of the uterus], there is nothing that Plan B is going to do to prohibit that. It's already happened," says Butler.

Plan B and ovulation

The main way plan B works to prevent pregnancy is to delay ovulation. It cannot stop the ovulation process if it's already started. Therefore, if you take plan B a day or two after you've started ovulating, it will be less effective.

How much less effective is unclear but according to one small 2011 study:

• Eight out of 45 women got pregnant if they took levonorgestrel on the day of ovulation or shortly after.
• No pregnancies occurred out of 103 women who took the pill before ovulation, showing that it is much more effective taken pre-ovulation.

It can be tricky to know if you've already ovulated or not if this isn't something you typically track. Ovulation happens about a week after the last day of your period and you might experience symptoms like:

• Thin, watery discharge
• Ovulation pain known as Mittelschmerz, which is when you feel one-sided pain depending on which ovary releases an egg

Despite being less effective if you've just ovulated, plan B also thins the uterine lining. So even if you have recently ovulated, you should still take it since your chances are better at preventing pregnancy than if you don't take it, Butler says.

Note: You can also consider a copper IUD, though you will need to schedule a doctor's appointment to get one. The copper IUD may be more effective at preventing pregnancy during or right after ovulation, since it may prevent implantation, even if fertilization has occurred.

Insider's takeaway

Plan B can be very effective at preventing pregnancy if you take it within 72 hours of unprotected sex, and preferably even sooner.

While you can take Plan B multiple times, it is meant to be emergency contraception and not a replacement for birth control.

If you find yourself using Plan B often, talk to your gynecologist about long-term contraception options.