Even when companies think they have taken the best precautions, it is inevitable that the drugs they sell occasionally will be misused or incorrectly prescribed. This is particularly the case with drugs indicated for serious conditions, where patients may be desperate for treatment.

Gilead Sciences Inc. ran into such a glitch last week when it was disclosed that two patients had suffered kidney toxicity potentially related to use of the company's Vistide cidofovir for cytomegalovirus retinitis. Despite the fact that Vistide was clearly contraindicated for these patients, GILD shares sank $3.50 to $25.25 on Tuesday amid investor worries that the news would affect sales. GILD closed the week at