But the implant's maker, Merck & Co., says the failure rate isn't exceptional, and that women should continue to use it.
Implanon contains synthetic progesterone and is inserted underneath the skin in the upper arm, where it is supposed to stop pregnancies for up to three years. More than 1 million implants have been sold since it was licensed in Britain in 1999.
On Wednesday, the Medicines and Healthcare Products Regulatory Agency said it had received more than 1,500 complaints of side effects — including pain and scarring — linked to Implanon since 1999. 584 women said they had suffered unplanned pregnancies.
Ronald Rogers, a spokesman for the New Jersey-based company, said the number of women in Britain who became pregnant were within the range the company would expect for that time period.
"We're very confident in the efficacy and safety of Implanon," Rogers said, adding it is more than 99 percent effective. He would not comment on whether other countries have reported higher than expected pregnancy rates in women who use Implanon.
Rogers said 600 pregnancies among Implanon's 1 million UK users translates into a rate of less than one percent. No form of contraception is 100 percent effective, he added.
Britain's healthcare regulator said in a statement that Implanon is a safe, effective and reliable contraceptive "when used correctly."
The agency recommended that any women worried about their contraceptive implants should use a condom as well for extra protection or talk to their doctor.
Copyright © 2011 The Associated Press. All rights reserved.
