Joe Zealberg, a psychiatrist in Charleston, S.C., prescribed generic drugs to patients for years and rarely had problems -- until last year. A number of patients who had done very well on brand-name medications "crashed and burned" when they switched to generics, he says.

One woman "went from being perfectly fine to crying inconsolably 24 hours a day" after she switched from one generic antidepressant to another, Dr. Zealberg says. Another patient was sold a generic version of his attention-deficit drug that contained no identifying...