In just one week, more than 120 people have landed in emergency rooms across the city because of bad reactions to synthetic marijuana, the Health Department said Friday.

The spike in ER visits occurred between April 8 and 15, with the majority of cases in East Harlem.

Synthetic marijuana — a plant-like material sprayed with chemicals to produce a high when smoked or ingested — is sold in grocery stores under street names like K2, Spice, Green Giant, Red Giant, AK-47 and “legal marijuana,” officials said.

It’s dangerous because of its unpredictable effects, including anxiety, confusion, agitation, rapid heart rate, seizures and fainting.

“That’s one risk, you don’t know exactly what you’re taking and exactly what effects it has on the body,” said Dr. Hillary Kunins, an assistant health commissioner.