Shares of Merck and Schering-Plough dropped greatly on Monday, after a research revealed that the cholesterol drugs the two companies produced were ineffective.

The companies started a very large advertising campaign in July 2004, after releasing Vytorin, which they had combined from ezetimibe and statin. Statins are a class of drug that reduce the amount of cholesterol produced in the body, and protect people from heart disease.

But partial results from a clinical study revealed that Vytorin was not more effective than Merck’s older drug, Zocor, which is sold for a fifth of Vytorin’s price.

The companies admitted that the combination between the two ingredients was less effective than statin alone, but US congressional committee is currently investigating whether the two companies were aware of the results for some time before announcing the public about the drug’s ineffectiveness.

Schering-Plough fell $5.37, or 28 percent, in New York Stock Exchange composite trading, the biggest drop since at least 1980. Merck fell 17 percent, which represents its biggest loss on the market since 2004, when the company had to withdraw its painkiller Vioxx.

Since the top U.S. heart doctors advised people with arterial problems not to use Vytorin anymore, or only use it as a “last resort,” prescriptions for the cholesterol drug fell 18 percent.

“This study provides no new evidence to support the use of this drug, and it moves us to more uncertainty about the benefits,'' said Harlan Krumholz, a cardiologist at Yale University, according to Bloomberg.com. “You've just seen a clinical trial that should change practice.”

Market analysts predict a prolonged fall for both companies in the next years.