Nearly two years after a clinical trial of the cholesterol-lowering drugs Zetia and Vytorin was finished, the results have still not been released by the drugs’ makers (Merck and Schering-Plough).

Yet, close to 800,000 prescriptions for the drugs are written each week in the United States, bringing in close to $4 billion for the year.

The companies have responded to cardiologists’ concerns over the lack of safety data for the drugs by promising to publish only a portion of the results in March, a move that critics are calling highly unusual.

Currently, no one knows whether Zetia and Vytorin are as effective as other cholesterol-lowering medications called statins. If they turn out to be less so, patients may be unnecessarily at risk of heart attacks.

Zetia was approved by the U.S. Food and Drug Administration in 2002 because it was found to lower LDL cholesterol by 15 to 20 percent. Vytorin combines Zetia with Merck’s statin drug, Zocor.

Together, the drugs have acquired close to 20 percent of the U.S. market for cholesterol-lowering drugs.

However, because Zetia and Vytorin work differently than standard statin drugs, cardiologists have expressed concerns that they may not protect the cardiovascular system as much as using a statin does.

Drugmakers Changed the Trial’s Endpoint

Along with announcing that they will only be releasing part of the clinical trial’s results, Merck and Schering-Plough said they had changed the trial’s endpoint, or the final result the study was supposed to measure.

Typically, the trial’s endpoint is supposed to remain constant because otherwise the study’s authors could change their goals to conform to the data they received.

The delay in publishing the study’s results raise further questions as to whether the drug industry is improving their disclosure of clinical trials.

Two years ago drugmakers promised to disclose study results publicly, however they face few penalties for failing to do so.