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The medication most commonly marketed as "Plan B," containing the chemical levonorgestrel, is an emergency contraceptive that can be used up to 120 hours after sexual intercourse to prevent conception. It's not an abortion pill like RU-486 (mifepristone). It won't cause the expulsion of a fertilized egg or an embryo from the uterus. Instead, it stops ovulation, keeping the egg from being released, and thickens the cervix, so sperm have to "swim" harder and can't reach the opening of the ovaries when fertilization ordinarily takes place. It used to be thought that Plan B could prevent a fertilized egg from implantation in the lining of the uterus, but the majority of scientific evidence now suggests that it works by preventing fertilization rather than by preventing implantation.

Because Plan B works by preventing ovulation and slowing down the travel of sperm to the reach the egg, the sooner it is taken after sexual intercourse, the more likely it is to work. Actually, it is even more effective when it is taken before intercourse. If it is taken in the first 24 hours after sex, it is 95 percent effective in preventing pregnancy. If a woman waits 72 hours, it reduces the risk of pregnancy by about 89 percent.

There is a long list of side effects that can be experienced in 10 percent or more of women who take Plan B:

  • Abdominal pain.
  • Nausea.
  • Headache.
  • Uterine pain.
  • Delayed menstruation.
  • Heavy menstruation.
  • Uterine bleeding.

There are also complaints that are less common, occurring in less than 10 percent of women who use the product:

  • Diarrhea.
  • Vomiting.
  • Painful menstruation.

Plan B is broken down in the liver by an enzyme called CYP3A4. The liver also uses CYP3A4 to process certain chemicals from food. If a woman eats foods containing the chemicals capsaicin or quercetin in excess on the same day as taking Plan B, there isn't as much CYP3A4 to process the levonorgestrol in the contraceptive so any side effects will be worse. Capsaicin is the chemical that gives hot peppers their heat. Hot sauce can interfere with the way the body processes Plan B. Quercetin is an antioxidant found in a variety of plant foods, but especially capers, cilantro, dill, Hungarian wax peppers, red onion, radicchio, watercress, fennel leaves, kale, cranberries, black plums, buckwheat, and sweet potato. Grapefruit juice, pomegranate juice, and noni also interfere with CYP3A4 and make Plan B side effects worse. It's only necessary to avoid these foods the day you take the contraceptive and the following two days. They won't cause any problems after the third day after you take Plan B. St. John's wort can make side effects worse. Valerian can reduce the effectiveness of the contraception. Unfiltered coffee also reduces the effectiveness of the contraceptive.

There's a very long list of medications that make Plan B more or less effective. If you have to take an antibiotic or an antidepressant or an epilepsy drug, the best thing you can do is to take Plan B as soon as possible, so it will be maximally effective, and brace yourself for side effects. You can always ask the pharmacist if Plan B interferes with your other prescriptions and what you should do to avoid problems.

What can women do to minimize side effects? It is usually better to take two doses of 0.75 mg of the drug 24 hours apart rather than 1.5 mg of the drug in a single dose. It's still important to take the first dose as soon as possible to prevent pregnancy, and it's also essential to follow up in 24 hours by taking the second dose. However, a woman's body can get a 0.75 mg dose into circulation at maximum concentration in just one hour, while it takes several hours for a 1.5 mg dose to get into the bloodstream in its fullest concentration. Women get slightly better protection from taking two small doses of Plan B rather than one large one.

What about "side effects" that seem to occur months after taking Plan B? These aren't side effects from the pill itself. Even spotting a week after taking Plan B probably isn't due to the Plan B. The only way to know for sure what is going on is if you have symptoms more than 48 hours after taking the emergency contraceptive pill is to see your doctor, preferably as soon as possible.

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