Duluth became the first Minnesota city to ban synthetic marijuana, making it illegal to sell, make, buy or possess the products, often sold as K2 or Spice, and often used as incense.

But the owner of the Last Place on Earth, a head shop, said the product accounts for half of his sales, the Duluth News Tribune reported. Jim Carlson has threatened to sue because the ban would place his business at an unfair disadvantage with competitors in neighboring communities, the paper said.

The city council approved the measure Monday night with no opposing votes, although one council member abstained, citing a conflict of interest because his radio station runs ads for the head shop.

The substance apparently doesn't show up on conventional drug tests.

The council also urged state officials to enact a statewide ban.

The state of Missouri has banned sales of K2, effective this week, but apparently the law doesn't cover other synthetic marijuana products.