The lawyer, Benjamin J. Horwich, said there was an exception for quantities of drugs that are “too insignificant or not important enough or too insubstantial.” But as a general matter, he said, prosecutors need only show that illicit drugs contributed to the victim’s death.

Requiring prosecutors to prove more, he said, would have pernicious consequences. “A ‘but for’ test,” he said, could mean that “nothing and nobody was the cause of the victim’s death.”

But Justice Antonin Scalia said the words of the statute might mean just that. “It says it has to result in the death,” he said of the law. “And I take that to mean at least, at least, but-for causality.”

The chief justice appeared to agree. “This statute does not say if you die from taking drugs, then the person who gave you drugs, who gave you any kind of drugs, is responsible,” he said. “It says the death has to result from the heroin.”

Justice Stephen G. Breyer urged caution, saying the court should not issue a broad decision in light of the complexity of the case, Burrage v. United States, No. 12-7515. “It’s so complicated to talk about,” he said, “that you are going to mix everybody up once we start to write on this.”

“Why not just say ‘substantial’ ” — meaning the mandatory sentence would be proper if the defendant’s drugs played a substantial role in the death — “and let the lower courts figure it out, so we don’t confuse the entire bar and the entire Congress and everything?” he asked.

Justice Scalia responded: “Because of that imprecision, some poor devils will have to go to jail for a longer period than otherwise. You know, tough luck.”