RARITAN, N.J., Feb. 17 (UPI) -- The U.S. Food and Drug Administration said it is monitoring conflicting data on the risk of thrombosis for women using the Ortho Evra birth control patch.

Two preliminary reports came to opposite conclusions about the non-fatal venous thrombosis risk of a transdermal contraceptive patch, compared with oral contraceptives, MedPageToday reported.

FDA officials said Friday they are monitoring the situation but they have planned no immediate action.

One epidemiologic study by i3 Drug Safety found that women who use Ortho Evra patch were not at increased risk for either heart attack or stroke; however, they had an about a two-fold increased risk for non-fatal venous thromboembolic events compared with women who took a norgestimate-containing oral contraceptive with 35 micrograms of estrogen.

Another study by the Boston Collaborative Drug Surveillance Program looked at the risk of heart attack, stroke and venous thromboembolic events in first-time users of the patch.

The study, published online in the journal Contraception, found that "the risk of non-fatal venous thromboembolic events for the contraceptive patch is similar to the risk for oral contraceptives containing 35 mcg of ethinyl estradiol and norgestimate."