Everything You Need To Know About Plan B Side Effects

When talking about Plan B, most people know the basics: It’s a pill you can take in the days following unprotected sex to help prevent pregnancy. But that simple understanding of the ‘morning-after pill’ doesn’t really explain what it actually does. Even people who have had experience using Plan B before might not know how it works and, in turn, why they feel certain symptoms after taking it. 

If taken correctly, Plan B is a form of emergency contraception that reduces your chance of pregnancy by 75-89%, according to Planned Parenthood. Like any other form of hormonal birth control, though, it does have some potential side effects. Luckily, these side effects aren’t serious.

To get the facts about Plan B side effects, we spoke with medical experts about what the morning-after pill actually does, how it affects your body, and why side effects might happen. 

First, it’s important to understand what Plan B is actually doing after you take it. “Plan B works by preventing ovulation, thus preventing the egg from being released by the ovary and greatly decreasing the chances of conceiving a pregnancy,” Dr. Meagan Haynes, MD, MPH, a board-certified OB/GYN and clinical professor at Yale School of Medicine, says. 

Plan B's active ingredient is levonorgestrel, which is a synthetic hormone and a kind of progestin. After you take Plan B, the progestin works to decrease the stimulation of two hormones: follicle stimulating hormone and luteinizing hormone. Usually, these two hormones work together to trigger the release of an egg, called ovulation. But the progestin in Plan B prevents this release in what's called negative feedback inhibition. “If it’s inhibiting those hormones, then it’s not going to stimulate that egg to release,” says Ashton Tureaud Strachan, DNP, FNP-C, a women's health nurse practitioner at Georgia Institute of Technology Health Services.

Remember, Plan B works to prevent ovulation, not fertilization. So if the egg has already been released, Plan B won't be effective — and it won't end an existing pregnancy. “It is not an abortifacient,” Dr. Haynes says.

Plan B is recommended for use within 72 hours of unprotected sex, but the sooner you take it, the better, as it becomes less effective over time. Another consideration to take into account is your weight, Dr. Haynes explains, as Plan B may not work for those who weigh more than 165 pounds. If that’s the case for you, IUDs or Ella might be better options. “Ulipristal acetate (Ella) is another oral emergency contraceptive available,” says Dr. Haynes. “It is slightly more effective than Plan B, with less than 1-3% pregnancy rates after use, though it requires a prescription and is less widely available.”

On the subject of other forms of contraception, Dr. Haynes also wants to remind people that Plan B isn’t the only option, despite being the most well known. “It is important to note that while many people have heard of the brand name Plan B, it is not actually the most effective method of emergency contraception available,” Dr. Haynes says. “The copper IUD (Paragard) and the 52mg Levonorgestrel IUD (Mirena and Liletta) when used for emergency contraception have pregnancy rates of less than 1%.” Plan B is still a good option, but it’s definitely something to keep in mind.

Now, the side effects.

First, Plan B can “potentially alter when your next cycle would be,” Strachan explains. Your period may arrive up to a week earlier or later than usual, or it could not be affected at all. It also may be lighter or heavier than usual.

Additionally, you may see some spotting between the time you take Plan B and your next period. Remember, for spotting, you might notice a few drops of blood on your underwear, but it won't look or feel like a normal period.

Generally, your period should return to normal by your next cycle, but if you take Plan B often, your period could become irregular for a longer time. That's part of why Plan B is a type of emergency contraception — it's meant to help prevent pregnancy if your regular form of contraception falls through, not serve as your primary form of birth control.

Clinical trials found that around 18% of people who use Plan B experience abdominal pain, which Plan B officially calls out as a common side effect. While that number seems small, it's still a possibility.

Plan B has a few other possible side effects, none of which are serious. Generally, Plan B is very safe and well tolerated by patients. The most common side effects apart from menstrual changes are “nausea or vomiting, lower abdominal pain, fatigue, headache, dizziness, or breast tenderness,” according to Dr. Haynes. 

Planned Parenthood notes that if you vomit within two hours of taking Plan B, it won’t be effective and you’ll need to take it again. Anecdotally, some people have noticed acne breakouts after taking Plan B, though it's not in the official list of side effects.

Apart from potentially throwing off the timing of your next period, Plan B side effects are usually pretty short-lived. Typically, most side effects are mild and only last for a day or two,” Dr. Haynes says. “The menstrual cycle usually returns within one week of the expected time. However, menstrual cycle changes can sometimes last up to a few weeks.” So if you are feeling Plan B symptoms two weeks later, it’s certainly not uncalled for.

If this all sounds familiar, that’s because the potential side effects of taking Plan B are similar to the potential side effects of taking other forms of hormonal birth control, like the pill. Which makes sense — both contain progestin, although Plan B contains a higher dose, and the pill usually contains estrogen as well (though there are some progestin-only forms of birth control pills). “It’s the symptoms you would potentially get if you took birth control, because this is also a form of birth control,” Strachan says.

“Someone should seek medical attention for any significant side effects or concerns for medication failure,” Dr. Haynes says. Via Plan B guidelines, if your period is more than a week late, then you should take a pregnancy test to make sure that Plan B was effective. And if you experience lower abdominal pain three to five weeks after taking Plan B, seek medical attention, because that may be a sign of an ectopic pregnancy. And as always, you should ask your doctor if you have any concerns.