Plan B One Step is the only form of emergency contraception available over-the-counter. Find out how it works, plus what side effects to expect and the cost of different Plan B pill options.

No matter what stage you're in when it comes to planning your family, it's possible that your primary form of birth control might fail, and you'll be considering a back-up plan. Thankfully, the "morning-after pill" is an effective emergency contraception option.

Here is what you need to know about Plan B One Step and its generic forms, Take Action, Aftera, and My Way.

What Is the Plan B Pill?

Plan B One Step and generic versions, commonly called the Plan B pill, is a form of emergency contraception made with levonorgestrel, the same progestin (a man-made form of the hormone progesterone) that's used in birth control pills. The FDA notes that Plan B works like other birth control pills to prevent pregnancy. More specifically, it prevents an egg from being released from the ovary (ovulation) and preventing the union of a sperm and egg (fertilization). Plan B and other morning-after pills do not end a pregnancy that has already implanted.

Plan B Pill Effectiveness

While concerns have been raised about morning-after pills being less effective in women whose BMI are higher than 25, or in some cases 30, the latest research has been mixed. Some studies—like one done by World Health Organization (WHO) researchers—say that BMI isn't much of an issue, while others—like a review of four available studies published in Contraception—warn that women who are considered obese might have a greater risk of pregnancy when using levonorgestrel-based morning-after pills.

The reason: "If I weigh more and my blood volume is higher, then I'm going to have a lower level of medication in my bloodstream," according to Cynthia Cork, M.D., an obstetrician-gynecologist in Fountain Valley, California.

Given the questions around BMI and Plan B, you might do well to explore emergency contraception options that are considered effective regardless of weight, such as the copper-T IUD (Paragard) and the prescription drug ulipristal acetate (ella), both of which can be more difficult to acquire, as they'll require a visit to a health care provider.

That said, as Planned Parenthood points out, you'll do best to take whichever form of emergency contraception you have access to, as that method will be better than using nothing at all.

Plan B Pill Side Effects

Your period might be delayed by up to one week, according to the Mayo Clinic. Planned Parenthood elaborates: "It may come earlier or later, and be heavier, lighter, or more spotty. Or it may be the same as it normally is."

Other side effects might include an upset stomach, lightheaded or dizziness, and tender breasts. If you throw up within two hours of taking the pill, it won't work, and you should take it again.

If you experience bleeding or spotting that lasts longer than a week or have severe lower abdominal pain three to five weeks after taking Plan B (which could indicate a miscarriage or ectopic pregnancy), see your health care provider.

When You Should Take the Plan B Pill

Plan B One Step works best when you take it within 72 hours, or three days, of unprotected sex, but you can take it up to five days after. That said, "The sooner, the better," says Cork. "If you just had intercourse last night, go first thing in the morning."

Plan B Pill Cost and Where to Get It

Since 2013, Plan B has been available with unrestricted for purchase over-the-counter at most drugstores and pharmacies. That means you can purchase Plan B or its generic forms right off the shelf, without showing ID and regardless of age or gender.

Plan B One Step usually costs about $50, while its generic counterparts are about $15-45. If you want to have one on hand should you need it in the future, you can order a generic form of the pill for $20 plus $5 for shipping at Some health insurance plans cover it, but you might need a prescription  in order for your insurance plan to pay for it.