American Home Products ( AHP) today named Robert A. Essner as the company's next chief executive, an appointment that will be effective May 1. Essner, who came to American Home Products in 1989, is currently the company's president, chief operating officer and a member of the board of directors. He replaces John R. Stafford, who will continue to serve as chairman. Essner, 53 years old, will remain the pharmaceutical company's president and will keep his position on the board. Shares of American Home Products, which is based in Madison, N.J., fell $1.89, or 3.1%, to $60.01 in recent New York Stock Exchange trading.