Shares of Schering-Plough ( SGP) and Merck ( MRK) gave up ground Monday after the companies announced neutral results of a study on their co-marketed Vytorin, a combination of cholesterol-lowering drugs simvastatin (Zocor) and Zetia. Schering-Plough shares fell $1.87, or 6.7%, to $25.86 in recent trading, while Merck was trading down $1.25, or 2.1%, at $59.30 after the companies' ENHANCE study, which was already a year-and-a-half delayed, showed no significant difference in the primary endpoints of the study. Rather than working to reduce cholesterol in the blood stream like most statins, Zetia targets cholesterol in the digestive tract -- and is intended with the capacity to complement statins like Zocor. The ENHANCE study found that while the pricier statin-plus-Zetia combination-drug, Vytorin, succeeds in the measure of lowering cholesterol, it had no more of an effect on reducing plaque buildup in the arteries than Merck's older statin, Zocor, which lost patent protection last year. ENHANCE involved 720 patients with heterozygous familial hypercholesterolemia (HeFH), a rare genetic disorder characterized by high levels of cholesterol and risk of cardiovascular disease that affects about 0.2% of the population. Eighty percent of the patients in the study had been previously treated with statins. Vytorin and Zetia, which cleared about $5 billion in annual sales according to Forbes, compete with the likes of Pfizer's ( PFE) Lipitor and AstraZeneca's ( AZN) Crestor. On the plus side, Vytorin did have a similar side effect and safety profile to the statin alone, according to the company. Merck and Schering-Plough are now conducting three large trials involving more than 20,000 high-risk patients.