A clinical trial that pitted two cholesterol-lowering drugs against each other provided strong evidence last week that one worked much better than the other in curbing the volume of artery-clogging plaque. Whether that matters much in helping patients prevent heart attacks and strokes remains to be demonstrated. Even so, the test is a welcome sign that drug companies may be increasingly willing to compare their own products with competitive drugs head-to-head. Assuming such studies are objective and thorough, they can provide patients and doctors with information that has long been unavailable.

For the most part, drugs in this country are not tested against other drugs in the same class. Instead they are tested against a placebo, and if shown to be comparatively safe and effective are approved for marketing. That leaves both patients and their doctors uncertain which approved drugs are better than their competitors and whether high-priced drugs warrant their added cost compared with lower-cost alternatives.

The clinical trial reported last week involved a class of drugs known as statins, which reduce the level of cholesterol-carrying low-density lipoproteins. The trial was sponsored by Pfizer and pitted Pfizer's Lipitor, the best-selling drug in the class, against Pravachol, an earlier statin made by Bristol-Myers Squibb. Some 500 heart patients with L.D.L. levels averaging 150 milligrams per deciliter of blood, far above the recommended 100, were given one drug or the other randomly. Lipitor was known in advance to be more potent and was given in bigger doses, so it was no great surprise that Lipitor cut the average L.D.L. level to 79 while Pravachol cut it only to 110. The potentially important finding was that Lipitor stopped the buildup of plaque whereas Pravachol did not, suggesting that current guidelines might need to be lowered.

There are reasons to be cautious in interpreting these results. Pfizer's sponsorship of the trial inevitably raises the question of potential bias, but the researchers, from the respected Cleveland Clinic, say they had complete control of the study, data analysis and preparation of a scientific paper. Most important, there is no proof yet that Lipitor is any better than Pravachol at reducing heart attacks and strokes, the end result of greatest concern to patients and doctors. Bristol-Myers Squibb itself is sponsoring a study that could begin to answer that question, presumably in hopes that Pravachol will prove as good or better at reducing disease than its more potent rival. Such studies are welcome if they are performed by reputable researchers given complete independence. But in the long run it would be best if the Food and Drug Administration were given the power to require head-to-head comparisons before approving a drug for marketing.