Merck and Schering-Plough said a closely watched study involving its Vytorin cholesterol treatment failed to reach its main goal.
The so-called Enhance trial tested Vytorin among patients with a rare genetic predisposition to dangerously high cholesterol levels.
The trial examined Vytorin -- which combines the companies' Zetia drug with Merck's older Zocor drug -- against Zocor alone in demonstrating plaque regression and cholesterol lowering, using a high dose of Zocor in each treatment group.
There was no statistically significant difference between treatment groups for the main goal, which was the change in thickness in the carotid arteries, the companies said.
But Vytorin did show it was significantly better at cutting "bad" LDL cholesterol, cutting it by 58 percent at 24 months, compared with 41 percent for just Zocor. Overall rates of serious side effects were similar, the companies said.
Vytorin and Zetia, whose combined sales reached about $1.3 billion in the third quarter, are critical products for both Merck and especially Schering-Plough.
Merck's shares lost more than 1.8 percent to $59.40 in early trading, and Schering-Plough shares fell 2.7 percent to $26.96, both on the New York Stock Exchange.
Natixis Bleichroeder analyst Jon LeCroy predicted that when the smoke clears, Schering-Plough stock will rally because no major safety problems with Zetia were seen in the trial.
Moreover, he noted that other big-selling cholesterol drugs have continued to fare well despite having failed to reduce plaque in arteries.