MINNEAPOLIS — The owner of a now-closed Minnesota head shop was sentenced to 17 1/2 years in prison Thursday for selling synthetic drugs in a case that prosecutors said should remove any doubts that substances sold under such names as “incense,” “spice” and “bath salts” are illegal.

James Carlson was convicted last October on 51 counts related to selling millions of dollars’ worth of synthetics from his Duluth shop, the Last Place on Earth, before authorities shut it down last summer after giving him repeated warnings that he was violating the law. Carlson never denied selling them, but disputed that his products were banned.

“This significant sentence should deliver a clear message to anyone in Minnesota who continues to sell synthetic drugs that they face serious criminal consequences,” Assistant U.S. Attorney Surya Saxena said outside the courthouse. “Further, this case should confirm once and for all that synthetic drugs are illegal.”

Carlson, 57, of Superior, Wis., gave the court a rambling statement more than 20 minutes long saying he felt unfairly singled out for prosecution. He argued that dozens of other stores in Minnesota alone and countless Internet sites across the country sell synthetics without interference from the government.

“Everybody admits the war on drugs is not working. It’s a fiasco,” Carlson complained.

U.S. District Judge James Doty rejected an argument that Carlson and defense attorney Randall Tigue had long made — that Carlson had good reason to believe the drugs he was selling were legal because of statements from the Drug Enforcement Agency and U.S. Sen. Amy Klobuchar bemoaning the difficulty of regulating them. The judge said the jury settled the question of whether his sales were illegal.

Doty also sentenced Carlson’s former girlfriend, Lava Marie Haugen, to five years in prison. She was convicted on four counts for her role in the operation.

The case was seen as one of the first major tests in federal court of how effectively authorities can combat synthetic drugs, which occupied a legal gray area that had been difficult for authorities to regulate because the makers constantly tweak their molecules to try to stay a step ahead of the law. The drugs mimic marijuana, illegal stimulants and hallucinogens.

City officials considered The Last Place on Earth to be blight on downtown Duluth. Scruffy customers would form long lines out the door. Hospital emergency rooms handled countless cases of people suffering ill effects from the drugs, such as paranoia, agitation, seizures and racing heartbeats. Police said one customer pulled out some of his teeth and another gouged his eye with a fork.

Duluth police statistics show that calls related to synthetic drugs rose steadily in the shop’s last year of operation, and fell off by 55 percent in the year after it closed. One of Duluth’s main hospitals reported a 95 percent drop in synthetic drug patients requiring emergency care after it closed.

Tigue told reporters they’ll appeal because Doty prevented them from fully explaining to the jury why Carlson believed he was acting legally.

“We’ll have an appeal filed by Monday,” Tigue said.

Saxena said he hopes the case dispels any doubts about whether synthetic drugs are unsafe.

“Whether they are sold in stores, on the Internet or on the street, these drugs are just as dangerous as traditional illicit drugs like methamphetamine, cocaine and heroin,” he said. “And their effects are often more unpredictable.”