The small, shiny packets that claim to hold only scented potpourri look harmless. One, a bubble gum variety of Scooby Doo Snax, bears a classically goofy image of the clue-sniffing dog. Another, Barely Legal, hints at naughtiness with a cartoon rendering of a woman’s torso.

But the police and city health officials say that the contents, drugs known loosely as synthetic marijuana, or K2, is wholly illegal and dangerous. A particularly toxic batch was responsible for a mass overdose in Brooklyn over the weekend, sickening at least 56 people and leading to at least 15 arrests since Saturday.

Investigators are working to determine the source of the drugs involved in the overdoses, and officials warned the public to exercise caution.

“K2 in and of itself is very dangerous,” Terence A. Monahan, the chief of department, said at a news conference at Police Headquarters in Lower Manhattan. “But what we’re seeing over the last couple of days — 56 confirmed overdoses — there is a very dangerous batch circulating right now in Brooklyn.”