Shares in Merck, the US pharmaceuticals giant, plunged by 27 per cent on Thursday after the company withdrew Vioxx, its $2.5bn-a-year painkiller, because of fears that it increases the risk of heart attacks and strokes.

Merck said it was taking the drug off the market voluntarily after a patient trial showed that those taking Vioxx were twice as likely to suffer cardiac problems than patients taking a placebo.