CHICAGO -- A prescription drug for yeast infections may increase chances for miscarriages when used early in pregnancy, a large study in Denmark suggests.

The 17-year study involved oral versions of fluconazole, available in pills or liquid. It doesn't provide any proof of harm, although the drug has been linked with other problems when used in high doses in pregnancy.

The researchers looked at more than 1 million pregnancies in Denmark, comparing outcomes among women who used oral fluconazole and those who didn't.

Women who used the drug during the first six months of pregnancy were almost 50 per cent more likely than nonusers to have a miscarriage. Women at increased risk included those on a low 150-milligram dose; those on higher doses faced greater miscarriage risks.

While fewer than 10 per cent of women in each group had miscarriages -- they totalled 710 -- the researchers said doctors should use caution in prescribing the drug early in pregnancy.

The study, by researchers at State Serum Institute in Copenhagen, was published Tuesday in the .

Yeast infections are common among women and occur more often in pregnancy because of hormonal changes. Over-the-counter creams and suppositories, and prescription fluconazole, in pills or liquid, are among treatments. Fluconazole is available in generic versions and under the brand name Diflucan, typically prescribed in a single dose. Multiple high doses of fluconazole are sometimes prescribed for stubborn infections.

A U.S. Food and Drug Administration safety announcement in 2011 said those higher doses in early pregnancy may be linked with certain birth defects but that a single 150 mg-dose appeared safe. Research since then on possible risks from low-dose versions has had mixed results.

Recent U.S. studies show that oral fluconazole isn't commonly prescribed during pregnancy.

Dr. Scott Sullivan, a member of the ethics committee at the American Congress of Obstetricians and Gynecologists, said the new study doesn't prove any miscarriage risk from fluconazole, "it just means we need more data, more studies."

Sullivan, an associate professor at the Medical University of South Carolina, said most oral medicines are used cautiously in women during the first 12 weeks of pregnancy because of concerns about potential risks.