The same drug combination behind July's outbreak of overdoses — fentanyl and a synthetic cannabinoid, commonly known as "K2" — was found in a sample of the drug that health officials suspected caused another overdose spike over the weekend.

Health officials said the combination turned up in a sample collected from Hahnemann University Hospital on Friday — at the beginning of a surge that would eventually sicken at least 110 people and kill 7 around the city.

Emergency room doctors said it was clear that many victims did not know what combination of drugs they were taking. And many displayed symptoms not typical of an opioid overdose: hallucinations, repeated vomiting, and severe agitation that, for some, required so much sedation, they had to be put on ventilators to breathe.

Those are symptoms similar to what many victims experienced in the July overdose outbreak, where overdoses spiked to 165 over a single weekend. 10 people died.

Synthetic cannabinoids are mind-altering chemicals considered far more powerful and unpredictable than cannabinoids found in natural marijuana. The synthetics are either sprayed on dried, shredded plant material so they can be smoked, or sold as liquids to be vaporized and inhaled in e-cigarettes and other devices, according to the National Institute on Drug Abuse.

Pat Trainor, a spokesman for the Philadelphia-area branch of the U.S. Drug Enforcement Agency, said it's difficult to track "bad batches" like the one circulating around Kensington last weekend.

Most fentanyl and most synthetic cannabinoids are produced in China; from there, he said, the trail gets fuzzier. It's unclear at what stage the fentanyl and K2 were mixed, he said, and unclear whether the people who packaged the drugs, or the dealers moving the batch on the street knew what they were selling.

"[The K2] is most likely a contaminant at a packaging-house level," he said. "And the people who are packaging it may not necessarily know what they're dealing with."

The DEA has offices in China that work with the Chinese government to track and halt the production of fentanyl and K2 — but an added challenge, Trainor said, is that both drugs have a number of analogs whose chemical compositions are slightly different — to skirt federal drug laws in both countries.

"But that's a pretty vast operation," he said. "And as the DEA, we certainly have an interest in tracking drug samples in our region: through drug purchases, seizures, intelligence from people who cooperate with us, and from people in the medical community. And, very sadly, we track drug trends through toxicology reports" — on victims of fatal overdoses.

Staff writer Mari Schaefer contributed to this article\