Several Supreme Court justices indicated strong skepticism yesterday about decisions in which federal trial and appeal judges have allowed the legal use Laetrile, a purported cure for terminally ill cancer patients.

In the key decision last July, the 10th U.S. Circuit Court of Appeals held that because the substance is intended for the dying, it is exempt from the requirement of the Food, Drug, and Cosmetic Act that anything classified as a "new drug" must be shown to be safe and effective.

When the exemption was defended yesterday by Kenneth R. Coe of Oklahoma City, counsel for a group of cancer victims, Justice Thurgood Marshall asked him: "Where do you get that from? The statute doesn't say it. The regulations do not say it."

Before Coe could answer, Marshall said-and the lawyer agreed-that the exemption came only frm the 10th Circuit, whose opinion was written by Chief Judge Oliver Seth of Santa Fe.

The decision permitted physicians everywhere in the country to administer Laetrile, which is derived mainly from apricot pits, intravenously to "certified" terminally ill patients.

Taken by mouth, the Food and Drug Administration has found, the substance is toxic in some persons; taken by injection, it hasn't been shown to be either toxic or nontoxic, Solicitor General Wade H. McCree Jr. told the court.

During the argument, the last of the current term, Justice Harry A. Blackmun inquired about the basis of the decision. "Common sense," Coe said. "I believe that's what the 10th Circuit was using. . ."

The appeals court ignored the pro-Laetrile decision made, on different grounds, by U.S. District Judge Luther L. Bohannon of Oklahoma City.

He had ordered the FDA to hold an administrative hearing. Commissioner Donald Kennedy did so and concluded that Laetrile-or various products using that name-was legally a new drug, having been found to be safe and effective neither by experts nor by acceptable scientific studies.

Kennedy also held that the substance hadn't been sold commercially before 1962 and therefore wasn't exempt under a "grandfather" clause enacted in that year.

Marshall asked if Bohannon hadn't simply thrown out Kennedy's findings "and substituted his own?

"That's exactly what he did," Coe said.

Bohannon held that Laetrile had been grandfathered. Yet, McCree told the court there was "no claim" that it had been marketed commercially before 1962.

The judge also declared the substance to be nontoxic.

What basis did he have for doing that? Chief Justice Warren Burger inquired.

He had "no evidence" for it, McCree said. The FDA, he pointed out, found that even in a dying person, a drug can be unsafe, because it can shorten life or aggravate symptoms. It also can be ineffective if it doesn't prolong life or provide the relief from pain claimed for it, he added.

Bohannon ruled that a constitutional right of privacy protected use of Laetrile.

Justice William H. Rehnquist said he found that curious. Having decided that Laetrile was grandfathered by the law, Rehnquist said, there was no need for Bohannon to reach a constitutional question.