Birth control implants release small amounts of progestin, a hormone that prevents ovulation and changes the consistency of the cervical mucus and uterine lining to make conception less likely. When inserted properly, they are 99.5 percent effective.
Birth control arm implants are effective 24 hours after insertion and last up to three years. After three years, the implant must be removed or replaced, and pregnancy can occur a few days after removal.
The Etonogestrel birth control arm implant is highly effective at preventing pregnancy. For birth control arm implants to be most effective, they must be inserted correctly and not paired with interacting drugs. Implant effectiveness in overweight women has not been studied.
Drugs that increase the metabolism of contraceptive hormones decrease the effectiveness of birth control implants. These include barbiturates, rifampin and St. John’s Wort, as well as drugs that treat HIV and yeast infections.
Implanon, which contains one rod and is the size of a matchstick, was approved by the FDA in 2006. The first birth control arm implant, Norplant, is no longer available.
Pregnancies that occur with the birth control arm implant have a high risk of being ectopic. Other risks include blood clots, ovarian cysts and irregular menstruation.