Shares in Schering Plough and Merck slumped on Monday after medical experts said their blockbuster anti-cholestorol drug produced in a joint venture by the companies was no more effective than cheaper generic versions.

A panel of heart doctors from the American College of Cardiology and a related editorial in the New England Journal of Medicine said that Zetia and Vytorin, crucial drugs for both companies, should be used as a last resort only if conventional cholesterol reduction therapies fail.