Your Best Birth Control

Injectable
What it is: A shot of progestin (brand name: Depo-Provera) given in the arm or butt once every three months. The primary users are teens and young adults who can't remember to take the Pill or don't want to leave clues for parents to find. Less popular now than it used to be (see below).
How it works: Same as the mini pill.
Caveats: Depo-Provera may increase the risk of osteoporosis when used for longer than two years. Also possible: weight gain-not just from water retention-of a few pounds a year, spotting, or the complete disappearance of your period. With this list, we don't blame you for looking for better ways to get progestin-only birth control (like the mini pill and implant) if you can't take estrogen.
Failure rate: .05 percent with perfect use; 3 percent with typical use. If you don't get the shot during the first five days of your period, use a backup method for at least a week.
Cost: $30 to $75 per shot.
How soon you can get pregnant: 10 months or more after stopping.

IUD
What it is: A small, T-shaped instrument inserted into your uterus. IUDs got a bad rap in the '70s because of the faulty Dalkon Shield, which caused thousands of infections (some fatal). But experts say the two versions now in the U.S.-ParaGard and Mirena-are safe when used properly (see caveats below). How it works: The ParaGard releases controlled doses of copper, which alters the fluids and environment inside the uterus, keeping his sperm from getting busy with your egg (you can leave it in for up to 10 years). The Mirena secretes small amounts of progestin for up to five years, thickening cervical mucus and reducing ovulation frequency. The former is best for people who want to avoid hormones-and who are OK with the fact that ParaGard can make periods heavier. The latter can reduce the flow dramatically, making it popular for women with menorrhagia. And while it contains progestin, little is absorbed into the bloodstream, so aside from some spotting in the beginning, side effects are rare. Caveats: IUDs occasionally slip out or push through the uterus during insertion. They can make an existing STD much worse. Insertion and removal are office procedures that may involve slight pain and cramping-not something you want to go through regularly-so they're best if you don't want children for several years.
Failure rate: For the copper IUD, .6 percent with perfect use; .8 percent with typical use. For Mirena, .1 percent with perfect use; same for typical use.
Cost: $175 to $500.
How soon you can get pregnant: Immediately after removal.

Our sources: Susan Loeb-Zeitlin, assistant professor of obstetrics and gynecology at New York Weill Cornell Medical Center; Hope Ricciotti, associate professor of obstetrics and gynecology at Harvard University Medical School; and Régine Sitruk-Ware, executive director of product research and development at the Population Council, plus the reproductive health gurus at Planned Parenthood and the Guttmacher Institute.