Medically reviewed by Erika Prouty, PharmD
There isn't a limit to how many times you can use the emergency contraceptive pill Plan B.
Here's the latest evidence about Plan B's effectiveness and some precautions to remember, along with a summary of other contraceptive options to help you make an informed decision.
How Effective Is Plan B?
In clinical trials, between 0.6% and 3.1% of people who used Plan B became pregnant. To put this in perspective, there is a 4% to 5% chance of becoming pregnant anytime you have sex, and this rate jumps to 30% around the time of ovulation.
Plan B is most effective when it's taken as soon as possible after unprotected intercourse. For it to work, it must be taken within 96 hours (four days) of sex. It may even work for up to 120 hours (five days), but more research is needed to know for sure.
The effectiveness of Plan B decreases over time. It's 5 times as effective at preventing pregnancy if it's taken during the first 24 hours after sex than it is after 96 hours.
Effectiveness also depends on the timing of the menstrual cycle. Plan B works if it's taken before ovulation occurs. Once an embryo has been implanted in the uterus, Plan B will not be effective, and the pregnancy will continue.
Plan B One-Step is a single 1.5 milligram (mg) dose of levonorgestrel, taken by mouth as soon as possible and within four days of unprotected sex. This product is sold in pharmacies under names like My Way, Aftera, and Take Action. These products are just as effective as Plan B One-Step.
The older version of Plan B contained two 0.75 mg levonorgestrel tablets to be taken twelve hours apart. This two-step product is no longer available in the United States.
If vomiting occurs within three hours of taking Plan B, you should take another Plan B pill.
If you have unprotected sex within 24 hours of taking Plan B, you don't need to retake it, but anytime after that first 24 hours, you should take another Plan B pill to be safe.
How Long Does Plan B Take to Work?
Plan B starts to work quickly. On average, it reaches the highest concentration in the body within about two hours.
A significant drawback to Plan B is that it is less effective for people with a body mass index (BMI) of over 25 and for those who weigh more than 154 pounds.
In one large study of females who took Plan B, those who weighed more than 165 pounds had about a 6% pregnancy rate vs. a 1.5% rate for those weighing less than 165 pounds.
Side effects associated with Plan B include:
Irregular menstrual bleeding (having your period earlier or later than expected or having spotting in between cycles)
Stomach pain or bloating
Note that Plan B may interact with prescription medications in several ways. Medicines like the following lower the amount of Plan B in the body and can decrease its effects:
Efavirenz, used for human immunodeficiency virus (HIV)
Carbamazepine, used for seizures
Oxcarbazepine, used for seizures
Phenytoin, used for seizures
Lamotrigine, used for seizures
Rifampin, used for tuberculosis
Saint-John's-wort, an herb used for depression
Plan B increases the amount of some medications and can increase the risk of their side effects. These drugs include the following:
Digoxin, a heart medication
Colchicine, used for gout
Fexofenadine, an antihistamine used for allergies
Plan B can be safely used during lactation, though a small amount does pass to the baby through breast milk.
What Are Other Birth Control Methods?
Birth control methods include emergency contraceptives and regular (ongoing) methods. There are many options depending on your needs and lifestyle.
Other emergency birth control methods available in the United States include intrauterine devices and the tablet Ella (ulipristal acetate).
However, the Food and Drug Administration (FDA) has not approved intrauterine devices. Here are the alternatives to Plan B One-Step:
Copper Intrauterine Device (IUD)
Copper IUDs are the most effective form of emergency birth control. The pregnancy rate with a copper IUD is very low, at 0.09%.
A healthcare provider must place these IUDs into the uterus within five days of unprotected sex.
The primary way copper IUDs work by impacting sperm movement. This prevents the egg from being fertilized. If an egg has been fertilized, a copper IUD may prevent it from implanting in the uterus. Copper IUDs can prevent pregnancies for up to 10 years.
This hormone-based IUD (brand names include Liletta and Mirena) gradually releases 52 mg of levonorgestrel over several years. A healthcare provider must place it in the uterus within five days of unprotected sex. It is not quite as effective as the copper IUD, though it still has a low pregnancy rate of 0.5%.
Keep in mind that if you become pregnant while an IUD is implanted, there is a possibility of the following risks:
Unlike emergency contraceptive pills, IUDs are not affected by body weight.
Ella (Ulipristal Acetate)
Ella is a single dose of 30 mg ulipristal acetate taken by mouth. It works similarly to Plan B, preventing or delaying ovulation. It is available only by prescription and can be taken up to five days after unprotected sex.
Ulipristal acetate is more effective than Plan B. The pregnancy rate for people who use this product is 0.9% to 1.8%. Studies have shown a 65% lower pregnancy rate for people taking ulipristal acetate than Plan B within 24 hours of intercourse.
Ulipristal acetate is affected by body weight, but not to the extent that Plan B is. It is equally effective in people with BMIs of up to 30, but people who are obese are twice as likely to become pregnant after taking it.
The Yuzpe method involves taking two doses of a combination birth control pill containing levonorgestrel and ethinyl estradiol. About 3% of people will get pregnant after using the Yuzpe method. It is likely to cause nausea and is less convenient than other oral (taken by mouth) emergency contraceptives.
Continuous Birth Control
Regular hormonal birth control methods are more effective than emergency contraception. These can include:
Oral contraceptive pills (birth control pills) that deliver estrogens and/or progestins
Ortho Evra, a hormone patch that is worn for three weeks each month
Nexplanon implant, a small rod inserted in the arm that works for up to three years
Depo-Provera, an injection given every three months
Nuvaring, which is inserted into the vagina and worn for three weeks each month
Intrauterine devices, inserted into the uterus for long-term protection
Barrier methods like condoms and planning based on your menstrual cycle (often called the rhythm method) have also been used to prevent pregnancy. Note the rhythm method, or fertility awareness methods, are between 77% and 98% effective and require great persistence and attention to detail to prevent pregnancy.
What Is the Most Effective Birth Control Method?
The copper IUD is the most effective method of emergency contraception, resulting in virtually no pregnancies if inserted within five days of unprotected sex. The levonorgestrel IUD has similar effectiveness. Remember that IUDs are typically more expensive and require an office or clinic visit for device placement.
In terms of the pills available, Ella is more effective than Plan B, but it requires a prescription and may not be readily available at all pharmacies.
Plan B One-Step is a single tablet of the hormone levonorgestrel, which prevents pregnancy by delaying ovulation.
It is most effective right after unprotected sex, and its effectiveness decreases over time. It won't work if it's taken more than four or five days after intercourse.
Several medicines, including some used for epilepsy, lower the effectiveness of Plan B.
It's best for people with a BMI less than 25 and less effective for people with higher BMIs.
Other options for emergency birth control are more effective, but they also have more barriers to use. These include copper and hormonal IUDs and the prescription Ella.
Frequently Asked Questions
How often can I take Plan B?
You can take Plan B as often as you need it. There's no evidence that using it often, even more than once in the same menstrual cycle, is dangerous.
How quickly does Plan B work?
Plan B works within three hours of taking it.
What’s the most effective birth control method?
The copper IUD is the most effective emergency birth control method. It prevents pregnancy in almost 100% of people who use it. Note that drawbacks associated with IUDs include an office or clinic visit is required for device placement by a healthcare provider and, depending on your insurance, the device and procedure can be expensive.
Read the original article on Verywell Health.