The pain-killer Darvon may stay on the market without new restrictions despite its reputation as a possible killer, Health, Education and Welfare Secretary, Joseph A. Califano Jr. ruled yesterday.

At the same time, however-in an unusual warning for the approved product-Califano asked doctors not to prescribe it, if possible. He also urged patients not to request it, or to ask their doctor for a substitute if it is prescribed.

Accepting the recommendation of his medical advisers, Califano decided to continue Darvon as a "Schedule II" drug. This means a doctor my let a patient refill a prescription for it as many as five times every six months.

Meanwhile, said HEW, Eli Lilly and Co., Darvon's maker, must conduct an educational campaign among doctors to try to limit Darvon's use. Darvon is currently the country's third-most-prescribed drug, despite many warnings about possible overuse.

Ralph Nader and his Health Research Group headed by Dr. Sidney Wolfe protested the HEW decision and urged doctors and patients "never to use this deadly and relatively useless drug."

They said suicides by Darvon users, accidental overdoses and misuse by drug addicts amount to 1,500 deaths a year. Thus, they said, the HEW decision means the sacrifice of the lives of "3,000 more people" before "the government" realizes that educational campaigns alone for abused drugs don't work."

Wolfe said he understands HEW intends to collect information and rely on education for two years before deciding whether to restrict Darvon further. Wolfe had petitioned FDA either to ban the drug or to move it to Schedule II, meaning tighter controls and no refills of prescriptions without a doctor's order.

Wolfe cited the fact that Darvon sometimes causes addiction; the belief of many authorities that it is not as effective a pain-killer as aspirin, acetominophen (Tylenol) or codeine, and the fact that many doctors and hospitals have abandoned its use.

Califano said there are still many unresolved questions about the extent to which Darvon contributes to suicides, accidental overdoses and addiction. As a result, he said he decided "not to take the extraordinary step of removing it from the market as an imminent hazard."

He ordered health officials to gather more facts. And, his public warning, which was almost as sharp as Nader's and Wolfe's, he said:

"Doctors should avoid prescribing Darvon whenever alternatives exist. Doctors should not prescribe cide-prone, who use alcohol heavily or who are taking other central nervous system depressants. They should warn their patients" about Darvon hazards.

"Pharmacists should warn patients of using Darvon . . . with tranquilizers, sedatives or alcohol."

"Patients should not ask for Darvon. If it is prescribed, patients should ask their doctor to prescibe one of the equally effective alternatives. Those who do take Darvon should have their doctor advise them of its dangers.