Pfizer Inc., the world’s biggest drugmaker, on Tuesday said patients taking its experimental drug to raise “good” HDL cholesterol experienced a slight rise in systolic blood pressure, as seen in earlier trials.

The company is developing a medicine that combines in a single pill the experimental drug torcetrapib with Pfizer’s top-selling Lipitor, which cuts levels of “bad” LDL cholesterol.

Pfizer said in a release that the study involved 493 patients receiving torcetrapib in various doses either alone or in combination with various doses of Lipitor.

The side effects of the drugs taken together were similar to those most commonly associated with drugs like Lipitor when taken alone, Pfizer said. But the company said patients taking the most commonly tested 60 milligram dose of torcetrapib with Lipitor also had an increase of 2 millimeters of mercury in their systolic blood pressure.

Researchers described the study, and two others involving the drug, at the annual meeting of the American College of Cardiology being held in Atlanta.

Systolic pressure - the “upper” number in a blood pressure reading - is the pressure of the contracting heart. Pfizer said the blood pressure increase will be “further defined” in ongoing late-stage trials.

Slight increases in blood pressure have also been linked to torcetrapib in earlier trials at higher doses of the drug, casting a cloud over the most important drug in the company’s pipeline.

On the favorable side, Pfizer said HDL levels shot up more than 54 percent among patients in a second trial who took a combination of 60 milligrams of torcetrapib and 20 milligrams of Lipitor each morning.

In a third trial of the combination product, Pfizer reported impressive benefits among those patients whose LDL levels were cut to levels below medical guidelines. Among such patients, each additional 1 milligram boost in HDL levels was associated with a 2 percent reduction in risk of heart attack or stroke.

The nutrient niacin, the standard drug currently used to boost HDL, has similarly been shown to reduce risk of heart attack and stroke. Although it typically raises the good form of cholesterol by 15 percent to 25 percent, use of niacin has been limited due to its tendency to cause facial flushing.

To skirt the flushing problem, Merck & Co is developing a product that would combine niacin, an anti-flushing agent and Merck’s LDL-lowering Zocor in a triple-combination pill.