Maxicare Health Plans' doctors are urged to prescribe generic drugs when possible. The result: About 40% of all prescriptions are now filled by generics--a $5-million savings last year in the HMO's drug purchasing budget.

San Diego allergist Dr. Eli Meltzer used to prescribe a brand-name drug called Theodor only for his asthma patients. But recently, he has been comfortable recommending a generic version of the medication for unclogging bronchial tubes. His patients save $12 a prescription.

Such growing acceptance by doctors combined with cost pressures on health care plans, such as HMOs, have catapulted generic drugs from about 5% of all

U.S. prescriptions 10 years ago to close to 40% today, according to drug analyst Hemant Shah of the HKS & Co. investment firm in Warren, N.Y.

Although dollar sales of generics have not grown as fast (because of the wide differences in price), the growing importance of generics in the U.S. health care system was demonstrated this week when Kansas City-based Marion Merrell Dow Pharmaceuticals--a maker of brand-name prescription drugs--announced that it is buying Rugby-Darby Group Cos., the nation's leading seller of generic drugs.

In April, Merck & Co., the world's largest pharmaceuticals manufacturer, announced that it would soon begin selling 10 discount-priced drugs via a generic drug subsidiary called West Point Pharma. Last year, Ciba-Geigy and Upjohn announced that they would begin marketing generic versions of certain brand-name drugs.

Marion Merrell Dow's decision was fueled in part by the fact that one of its major products, Seldane, the nation's leading anti-allergy drug, will lose its patent protection in 1994. Once the patent expires, the drug will be available for the generic market.

Moreover, the company last year lost its exclusive patent on Cardizem--a drug to treat high blood pressure and angina--which generated 32% of the company's 1992 sales. And no new blockbuster drugs are on the horizon to compensate for those patent losses, said Jack Lamberton, a drug industry analyst with NatWest Securities in New York City.

Generic drugs may differ somewhat from the brand-name versions, but they must receive "bio-equivalent" certification from the U.S. Food and Drug Administration before they can be marketed.

Because manufacturers spend a minimum on marketing and research, the drugs on average sell for half the price charged by former patent holders.