A top Merck executive testified yesterday that the Vioxx manufacturer feared as much as a $500 million drop in sales if regulators had succeeded in placing an unfavorable warning on the Vioxx label three years ago. David Anstice,who headed Merck’s US marketing campaign at the time, admitted an internal Merck memo showed the company feared a huge drop in sales if the FDA won its battle over the Vioxx label.