Women with severe migraine headaches with aura or have any migraine headaches if they are over age 35 should not use the contraceptive device NuvaRing, according to Merck, its manufacturer.

NuvaRing, is a small vaginal ring that releases hormones that prevent pregnancy. Earlier this year, Merck agreed to pay $100 million to settle lawsuits brought by more than 3,800 women in the United States, alleging that the company failed to warn about the risk of developing blood clots when using the device. Merck indicated at the time of the settlement that it stands by the safety of the ring.

In July, however, after consulting with Merck, Health Canada announced new restrictions for using NuvaRing. The  statement indicates that women who smoke over 35, have migraine, heart problems, diabetes, high blood pressure, abnormal levels of blood-fat, clotting disorders, or have a limited ability to move around for a long time after an operation should not use NuvaRing.

A public announcement was made in Canada about that new warning, but a similar announcement apparently was not made in the United States. On the NuvaRing website, however, the cautionary information appears under the tab “Who Should Not Use It.”

Studies evaluating NuvaRing and the development of blood clots have been contradictory. A study Merck funded found the risk of developing clots while using the ring to be similar to that of the risk when using oral contraceptives. A study by the United States Food & Drug Administration found no difference in risk among new users of low-dose combined hormonal contraceptives, the NuvaRing, or the birth control patch. A study published in the British Medical Journal, however, found that women using the NuvaRing were about six times more likely to develop blood clots than women who did not use any form of hormonal contraceptive and about twice as likely to develop clots as women taking oral contraceptives with levonorgestrel, a synthetic progesterone.

National Headache Foundation President Dr. Arthur Elkind noted that although the studies from the United States differ from the Canadian and British reports regarding safety of the device, he recommends that individuals suffering from migraine with aura or complicated forms of migraine use alternate types of contraception until the questions about NuvaRing’s safety can be completely resolved.