CHICAGO (Reuters) - Doctors should prescribe the lowest doses of Celebrex possible in patients at high risk of heart problems, researchers who did a combined analysis of six studies of the Pfizer Inc pain drug said on Monday.

The analysis suggests that the potentially harmful effect of Celebrex dosage is most pronounced in higher-risk patients, researchers said at the American College of Cardiology scientific meeting.

"These data should provide some measure of comfort in prescribing celecoxib to patients with very low cardiovascular risk," said Dr. Scott Solomon, lead researcher of the National Cancer Institute-sponsored study, using the generic name for Celebrex.

"Similarly, we should be cautious in prescribing celecoxib to patients who have elevated baseline cardiovascular risk," said Solomon, whose study was also published online in the journal Circulation.

Celebrex belongs to a class of pain drugs known as cox-2 inhibitors and is the only such drug still on the market in the United States. Merck and Co withdrew its widely used cox-2 drug Vioxx from the market in 2004 after a study showed it doubled the risk of heart attack and stroke in long-term users. Another cox-2 inhibitor from Pfizer called Bextra was also pulled from the U.S. market over safety concerns.

The six trials involving a total of 7,950 patients all studied Celebrex, known chemically as celecoxib, against a placebo for uses other than arthritis. Patients received either 400 milligrams once a day, 200 mg twice a day, or 400 mg twice a day.

The primary goal of the pooled analysis was an assessment of the combined risk of cardiovascular death, heart attack, stroke, heart failure or events involving blood clots.

Researchers found that the heart risk was lowest at the 400 mg daily dose. They found nearly a two-fold risk of adverse heart events with the 200 mg twice daily dose and that jumped to about a three-fold risk at the 400 mg twice daily dose.