CHICAGO (Reuters) - Pharmacies could be more vigilant about preventing combinations of drugs that should not be taken together from being dispensed, researchers said Tuesday.

Computerized warning systems now in use should enable pharmacists to identify descriptions containing contraindicated drugs, or medications that regulators have warned should not be taken together, researchers for a drug safety group said.

A study of a heartburn medication that was later withdrawn from the market found that patients who received prescriptions for it and a contraindicated drug most often got both medications from the same pharmacy.

The drug, cisapride, was voluntarily withdrawn from the market last year after it was blamed for causing an irregular heartbeat in 270 patients, 70 of whom died. Most of the cases involved use of cisapride with contraindicated drugs.

Of more than 131,000 cisapride prescriptions examined between 1995 and 1999, more than 3 percent, or 4,414, were dispensed with at least one contraindicated drug. Of those, 50 percent were prescribed for patients by the same physicians and 89 percent were dispensed by the same pharmacies -- 17 percent of them on the same day.

Study authors Judith Jones and Suellen Curkendall of The Degge Group Ltd., in Arlington, Virginia, writing in the Journal of the American Medical Association, said they did not determine how often pharmacies caught the mistake in prescribing bad drug combinations, but they suggested that warning systems could be improved without resorting to complicated communications among different pharmacies.

"This suggests that the pharmacist, who is focused solely on the therapy, more often has the opportunity to apply critical information necessary to prevent contraindicated co-dispensings," the report said.