By Debra Sherman

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 (PFE.N: Quote, Profile, Research) pain drug said on Monday.

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

"Patients at low cardiovascular risk should take some level of comfort," said Dr. Scott Solomon, lead researcher of the National Cancer Institute-sponsored analysis. "That wasn't so clear previously."

Doctors should exercise caution in prescribing the drug, generally known as celecoxib, in high-risk patients, said Solomon, whose analysis 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 Inc (MRK.N: Quote, Profile, Research) 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 due to safety concerns.

While the analysis sheds light on how the drug should be used, Solomon said he could not judge whether the data lift the cloud over this class of drugs.

The six trials involving a total of 7,950 patients all studied Celebrex 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.