Nearly 600 UK women who used the Implanon contraceptive have had unwanted pregnancies, even though the tiny device - which is fitted into the upper arm - is touted as being able to prevent conception for up to three years. Their concerns have been debated for months on discussion boards, such as this one and this one, where women share side effect stories and advice.

The Merck device has been used for more than a decade, but more than 1,600 women have complained about a host of problems, including scarring, incorrect fittings and outright failure, to the UK’s Medicines and Healthcare products Regulatory Agency, which today issued a safety update. In all, more than 2,800 side effect reports have been registered with the agency, which last fall pressured the drugmaker to replace Implanon with a newer version called Nexplanon that is designed to reduce the risk of insertion errors (see here).

The MHRA attributed most of the problems to improper fittings. For its part, Merck tells The Guardian that it remains confident about the safety and efficacy of its device. “The basis for successful use of Implanon is a correct and carefully performed subdermal insertion of the implant in accordance with the product instructions. If the implant is not inserted in accordance with the instructions and on the correct day, this may result in an unintended pregnancy. In addition, no contraceptive is 100 percent effective.” Of course not, although Merck is happy to boast on the Implanon web site that the device is more than 99 percent effective. But why quibble?

pic thx to 3neus on flickr