The new requirement is one of several steps the agency said it was taking to fight medical errors, which claim tens of thousands of lives a year in the United States.

The food and drug commissioner, Dr. Mark B. McClellan, estimated that the bar-code requirement, introduced as a proposal expected to gain final adoption after a 90-day period of public comment, would prevent 400,000 bad drug reactions - from headaches to death - over the next 20 years.

Under bar-code systems, already in use at veterans' hospitals and some others, doctors enter prescriptions for hospitalized patients into a computer, which checks those prescriptions against parameters like the patient's age, weight, diagnosis and other drugs taken.

Then, at bedside, a nurse scans a bar code on the patient's bracelet and a bar code on the medication. The computer instantly reads whether the medication is the same drug, with the same dose, dispensing time and delivery method, that the doctor ordered, and sounds an alarm if there is a mismatch.

Medical safety experts, patient care groups and associations representing hospitals, pharmacists and drug companies all greeted the new rule with enthusiasm.

"I'm really happy about this," said Michael R. Cohen, president of the Institute for Safe Medication Practices, a consumer safety group. "Early studies show massive reductions in errors, over 50 percent reductions."

Dr. David W. Bates, a Harvard Medical School professor who studies patient safety, called the announcement "an enormously positive development."

The F.D.A. also announced that it was revamping its system for reporting bad medication reactions and bad reactions to blood or blood products.

Under the new drug-reaction rules, minor reactions must be reported more promptly by pharmaceutical companies, which will also be required to report even "near misses," such as a wrong drug's being nearly given to a patient because of confusion over similar medication names.

Blood banks and blood product makers will have to report all suspected serious reactions, not just fatalities as at present.

As for bar-coded dispensing, the Veterans Health Administration, an agency of the Department of Veterans Affairs, is a pioneer, having built a nationwide pharmacy system using drugs ordered by computer, packed by robots and checked at the bedside of veterans' hospital patients by nurses using bar-code scanners.

Dr. Jonathan B. Perlin, an official of the agency, said some of its 150 hospitals now had "virtually no dispensing errors," in contrast to findings of previous studies that, he said, showed that one in seven hospitalizations nationally was complicated by a bad reaction to a drug.

The V.A. prints its own bar codes, but that would be too expensive for most hospital pharmacies. The F.D.A., which regulates drug makers, not hospitals, will order the pharmaceutical companies to come up with codes identifying each drug and dosage. Whether lot numbers and expiration dates will be included is still under consideration.

The Food and Drug Administration estimated that it would cost pharmaceutical companies $50 million to put bar codes on every product and that hospitals would spend over $7 billion on scanners and computers.

Hospital groups acknowledged the high price to them, but said enhanced patient safety would be worth the cost.

"There is a concern that it will cost a lot of money," said John Combes, a spokesman for the American Hospital Association, who represents the organization on hospital safety panels. But because it protects patients, "there's a lot of interest in getting this going," he said.

The Institute of Medicine of the National Academy of Sciences estimated in 1999 that medical errors killed as many as 98,000 people a year in the United States, more than car crashes. About 7,000 of those deaths were attributed to drug errors, including wrong drugs, wrong doses or fatal combinations. A 2002 study of 368 hospitals blamed overworked nursing staff, doctors' handwriting and computer-entry errors for drug mistakes.

Pharmacists have reported many mix-ups in drugs with similar names, like the anti-convulsant Lamictal and the anti-fungal Lamisil, or the antihistamine Zyrtec and the ulcer drug Zantac. The most common hospital mistakes are doses missed because a patient is out of the room, said Mr. Cohen, the consumer advocate. But patients have been given wrong drugs because they have names like other patients, because a nurse has handed out drugs without realizing a previous nurse did so, or because doses get mixed up.

Bar codes now appear on only about 35 percent of all hospital medications, which are typically packaged in single doses of a pill or two or in a single syringe ampul. The UPC bar codes used in supermarkets are too big for those packages, and drug companies have had difficulty settling on a standard, a representative of their trade association said. And without the codes, hospitals have been reluctant to buy scanning equipment.