The U.S. Food and Drug Administration said on Monday it had asked German drugmaker Bayer AG to conduct further studies on its implantable sterilization device, Essure, following thousands of complaints and calls for its withdrawal.

Essure, acquired by Bayer in 2013, consists of two small nickel-titanium coils inserted into the fallopian tubes. Scar tissue forming around the device prevents pregnancy.

Bayer estimates there are about 750,000 women using it worldwide, about 70 percent of them in the United States.

The device, approved for sale in 2002, was billed as an alternative to tubal ligation. However, the FDA has received thousands of complaints over the years, including reports of the device breaking or moving and causing injuries.

The FDA has instructed Bayer to conduct a clinical study to determine heightened risks for some women and has issued draft guidance with labeling recommendations, including a "boxed warning label" - the most restrictive type of warning.

The agency also issued a checklist for doctors to use when discussing the risks of implanted permanent birth control devices. 

Personal injury lawsuits filed against Bayer have claimed the implant had caused allergic reactions, severe pelvic pain and required surgeries to stabilize or remove the device.

More than 5,000 adverse events involving Essure have been reported, according to the FDA's website, although the role of the device is not always clear.

The FDA also ordered Bayer to conduct a study designed to provide information on the "real world" risks of using the device.

In September, the FDA called a meeting of its advisory panel to weigh the benefits and risks of Essure following complaints that it had caused life-altering side effects, from chronic pain and bleeding to autoimmune disorders such as psoriasis and lupus.

The FDA said on Monday that while it believed Essure was an appropriate option for most women seeking a permanent form of birth control, some may be at risk of serious complications.

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Bayer said it would work with the FDA, while reiterating the positive benefit-risk profile of Essure.

Women implanted with the device are more than 10 times likely to require post-procedure surgery than those who undergo laparoscopic sterilization, according to a study published in the British Medical Journal in October.

A private analyst who combed through the FDA's public database said this month the agency may have greatly underestimated the number of fetal deaths among women who became pregnant after using Essure.