In a statement, Mr. Huffman said the chemicals were not intended for human use. He added that his lab had developed them for research purposes only, and that “their effects in humans have not been studied and they could very well have toxic effects.”

Nevertheless, pure forms of the chemical are available online, and investigators believe that many sellers are buying bulk quantities, mixing them with a potpourrilike blend of herbs and labeling the substance K2.

“It’s not like there’s one K2 distributor  everybody is making their own stuff, calling it K2 and selling it, which is the most unnerving aspect,” said Dr. Christopher Rosenbaum, an assistant professor of toxicology at the University of Massachusetts who is studying the effects of K2 in emergency room patients.

The American Association of Poison Control Centers reports that so far this year there have been 567 K2-related calls, up from 13 in 2009. But investigators add that no one is really certain what is in K2, and people are arriving at emergency rooms with symptoms that would not normally be associated with marijuana or a synthetic form of the drug.

“I don’t know how many people are going for a box of doughnuts after smoking K2, but they’re sure getting some other symptoms,” said Dr. Anthony Scalzo, a professor of emergency medicine at the St. Louis University who first reported a rise in K2-related cases and is collaborating with Dr. Rosenbaum in researching K2’s effects. “These are very anxious, agitated people that are requiring several doses of sedatives.”

Dr. Scalzo, who is also the medical director for the Missouri Poison Control Center, added that although tests had found cannabinoids in K2, it was unclear “whether the reaction we’re seeing is just because of dose effect, or if there’s something in there we haven’t found yet.”

That question remains at the center of an investigation into the death of David Rozga, an Iowa teenager who last month committed suicide shortly after smoking K2. Mr. Rozga, 18, had graduated from high school one week earlier and was planning to attend college in the fall.