The Food and Drug Administration yesterday approved for sale a drug that lowers cholesterols and is expected to help reduce the risk of heart disease in millions of people.

The drug, called lovastatin and sold under the brand name Mevacor, is one of a new class of cholesterol-lowering agents that doctors say will assume an important role in preventing strokes and heart attacks.

It will be used to treat high cholesterol levels in the 400,000 Americans who have an inherited condition of high cholesterol that is unaffected by changes in diet and exercise.

The drug also may be prescribed for others with "severe" cholesterol problems that are not helped by at least three months of adjusted diet and exercise levels, according to Dr. Antonio Gotto of Baylor College of Medicine, speaking for the drug's maker, Merck, Sharpe and Dohme.

Gotto said there may be several million people in that category. He also said that there is some potential for abuse with the drug, if doctors prescribe it casually to lower cholesterol without first trying to reduce the levels by diet and exercise.

The drug must be taken daily for life to keep cholesterol levels down. But studies show that it is more effective and apparently has fewer side effects than other medications.

It can reduce by 30 percent to 40 percent the most dangerous form of cholesterol, the LDL or low density lipoprotein cholesterol, the company said. Current medications commonly reduce the levels by 20 percent.

The side effects of the new compounds include impaired liver function, muscle inflammation and visual impairment. The liver problems showed up in about 2 percent of the 1,700 people studied over two years before the drug's approval. It is not known whether more side effects will show up when people take the drug daily for many years.

"The drug may prove a useful addition in the fight against coronary heart disease, our No. 1 cause of death," said FDA Commissioner Frank E. Young in a statement. "High cholesterol, along with cigarette smoking and high blood pressure, is a primary factor predisposing Americans to heart disease."

Because of the effect on the liver and eyes noted in people participating in clinical trials, patients who receive lovastatin should have their blood tested for liver function every six weeks and have an eye test once a year, the FDA said.

Gotto said he would prescribe the drug for adults over 40 whose cholesterol level is over 260 milligrams per deciliter, if they could not reduce their cholesterol by other means. Ninety percent of Americans have levels lower than that, with 215 as the mean level for the nation.

For those 20 to 40 years old, he said, he would recommend use with a cholesterol level of 240 milligrams or over.

Cholesterol is a fatty substance that the body manufactures and takes in as food. It is an essential nutrient. But excessive levels can leave deposits on the walls of blood vessels. These can build up to the point that they choke off the vessels, thus causing strokes in the brain or heart failure when they occur in the heart.

In the body, cholesterol is made by a sequence of chemical steps. In one key step, a large molecule is snipped by an enzyme into a smaller precursor of cholesterol. The drug acts to block the action of that enzyme and stops the chemical cascade that produces cholesterol.