More than a third of patients given drugs to combat high blood pressure and control cholesterol stopped taking the medicine after 6 months, a study said on Monday.

The heaviest fall-off was among patients who were already taking a number of prescription drugs, perhaps reflecting the complicated task of managing several drugs at once.

On the other hand, patients who started both the hypertension and fat-lowering drugs at, or near, the same time were more likely to continue taking both of them, than when there was a longer gap separating the beginning of the two therapies, it added.

Lowering fat in the blood stream and keeping blood pressure under control are key to preventing Heart disease.

Researchers at ValueMedics Research, L.L.C., in Arlington, Virginia, a private firm that does consulting for the drug industry, said they looked at 8,406 patients who had been prescribed both types of drugs during a 90-day period from 1997 to 2001.

The percentage of patients taking both drugs as prescribed declined sharply over the course of the study. After six months, 35 percent were taking neither prescription, while 44 percent were still taking both. The rest took one drug, but not the other.

After 12 months, more than 35 percent were still taking neither medication, while the number taking both prescriptions had fallen to about 36 percent. The rest took one drug, but not the other. The breakdown was about the same after three years.

The authors suggested doctors check back within the first six months to make sure patients are taking their medication. They also said previous studies have suggested “that simplifying a drug regime by eliminating even one pill” could improve adherence among users faced with taking several.

Patients who started the two drugs on the same day, or within one month of each other, were 34 percent more likely to continue taking both medications. In general, women were more likely to follow dosage directions than men.

The study, published in the Archives of Internal Medicine, was based on insurance company medical records.