Madison police are concerned about strange and violent behavior they say is caused by users of synthetic marijuana.

Officers said they’ve seen violence, seizures, and in one case, a heart attack they believe is connected to synthetic marijuana use.

Two people were arrested near a bus shelter Wednesday for dealing or using hazardous substances. Police said the two were sharing a cigarette that one admitted was “spice,” a name given to herbs sprayed with lab chemicals. The products were purchased at local businesses, and are often sold as herbal incense. Packages are labeled “not for human consumption.”

One of the two told police he was aware of the dangers but that he’s hooked.

When smoked, synthetic marijuana is said to give users a stronger high than what they would get from using real marijuana. Researchers have learned ingesting such chemicals can lead to seizures, hallucinations, convulsions and extreme psychological problems, police said.

Police said users are known to congregate in the upper end of State Street.

Madison crime stories from the past 12 months - locations are approximate