LINCOLN, Neb. (AP) — The city of Lincoln is threatening legal action against stores that sell synthetic marijuana unless they agree to stop carrying the product.

Mayor Chris Beutler announced Thursday that the city attorney sent letters to five known Lincoln retailers that sell the drug, commonly known as K2.

Lincoln public safety and health officials say more than 100 people have overdosed on synthetic marijuana in the past couple of weeks. Members of law enforcement say the chemical compounds cause violent and unpredictable reactions.

K2 is legal unless smoked or otherwise ingested.

Letters were sent to Island Smokes, Wicked Smoke, Dirt Cheap and Aladdin Tobacco’s two Lincoln locations.

City Attorney Jeff Kirkpatrick says he hopes to have the letters signed by May 15.