Major drug companies are boosting their year-end stockpiles in case of hoarding by people afraid that Y2K computer troubles will cut off their supply of life-sustaining medication.

Drug manufacturers and pharmacies say they have fixed their computers to correctly read the date in the new year. Their main fear instead is overreaction by senior citizens, diabetics and other patients.

"There is a huge supply of prescription medicines, but there isn't an unlimited supply," said Robert Grupp, spokesman for Eli Lilly & Co. "Hoarding in one area certainly has the potential to cause some shortages in another."

Pharmaceutical companies are also hoping the stockpiling itself discourages hoarding.

"By virtue of knowing we have more than adequate supplies, that should give our customers confidence that we can meet any extraordinary demand," said Tom Fuldner, a spokesman at Glaxo Wellcome.

Glaxo Wellcome boosted inventories of its drugs by roughly 15 percent. Eli Lilly, the No. 1 maker of insulin in the United States, will have a 45-day supply on hand, rather than the normal 30-day reserve.

Bristol-Myers Squibb bought extra raw materials, identified alternative suppliers and kept stocks of partially completed drugs so it can increase production if needed.

Critical care products, such as those for cancer and AIDS, got special consideration.

At the same time, the pharmaceutical industry is trying to discourage overbuying, recommending that patients get regular refills five to seven days before their medication runs out, just as they normally should. Manufacturers believe they can get drugs to patients within that amount of time.

"There could be minor inconveniences from place to place, but those inconveniences could be overcome," said Bristol-Myers spokesman Patrick Donohoe.

Drugmakers said that if they have to, they will be able to get medication through methods reserved for hurricanes and other disasters: express mail, chartered planes and emergency deliveries by police.

William K. Hubbard, senior associate commissioner at the Food and Drug Administration, said the agency audited manufacturers of the 200 best-selling drugs, as well as companies that are the sole supplier of a specific treatment.

He said that he is confident manufacturers will be ready and that the agency has found no evidence that consumers are stocking up.

Insurance companies and doctors generally limit how much medication a patient may get at a time. Any significant stockpiling would probably have to occur over many months.

For the most part, the industry keeps 90 days' worth of drugs on hand, so drugs for Jan. 1 have already been manufactured. And although more than half of raw ingredients come from abroad, where Y2K troubles could be worse, supplies needed for early next year have largely been delivered already.

Despite the confidence, health care experts offer some tips for consumers:

* Maintain a list of medications used by each member of the family, along with dosage and the name of the prescribing doctor.

* Document important medical information, such as drug allergies.

* Carry insurance cards.

Sarah Datz, a spokeswoman for the Rite Aid pharmacy chain, said pharmacists are trained in filling prescriptions in emergencies, even if the power goes out and computers go down.

CVS Pharmacy aired TV commercials in November to stress its Y2K readiness and distributed brochures to customers.

John Koskinen, President Clinton's Y2K chief, said the pharmaceutical industry has withstood past emergencies.

"In all the natural disasters in the last 20 years, nobody has had to do without prescription drugs for more than 24 to 36 hours," Koskinen said. "The only thing that could sink the system would be overreaction."