Discusses how researchers are finding that smaller-than-usual doses of a wide range of drugs can save people unpleasant side effects--and money. Number of drugs that are sold with information for doctors on `low dose' use; Benefits of low doses of Prozac, lovastatin, and others; Research specifics; Marketing potential; Launch of new lose-dose drugs; Ziac for high blood pressure; Estrace, a preventative drug for osteoporosis. INSET: Doses based on weight, not age, by D.P..