A week before actor Philip Seymour Hoffman overdosed on a mix of heroin, cocaine and other drugs, the Supreme Court restrained what one top prosecutor called "the strongest tool" federal authorities have to go after dealers in such cases, and now some U.S. drug prosecutions are getting sent to rehab.

"We may not be able to meet the standard of proof in those cases," the U.S. Attorney in Vermont, Tris Coffin, said of overdose cases involving a cocktail of drugs....