The owner of a onetime Duluth head shop and two shop employees have lost a federal appeal of their convictions for selling synthetic drugs.

An Eighth District U.S. Court of Appeals panel ruled Thursday that convictions should stand for former Last Place On Earth store owner James R. Carlson, along with his girlfriend Lava M. Haugen and his son Joseph J. Gellerman, both of whom worked at the shop.

The defendants had challenged the constitutionality of the federal Controlled Substance Analogue Enforcement Act, under which it is against the law to knowingly or intentionally manufacture, distribute or dispense a controlled substance analogue, which has a chemical structure and effect that is substantially similar to an illegal drug.

The U.S. Supreme Court recently determined that the act is not unconstitutional, the court wrote.

In Carlson and Haugen’s cases, the appeals court found evidence that both knew the chemical structures of the analogues sold were substantially similar to controlled substances. The court cited an instance in which Carlson “stated to the media that the government could not keep up with policing synthetic drug distribution because he believed that his conduct was legal as long as the substances he sold were ‘one little molecule’ different from banned controlled substances.”

Carlson’s attorney, Mark Nyvold, declined to comment on the decision and said he would talk with his client about whether to pursue further proceedings.

Carlson, now 58, was convicted in October 2013 on more than 50 federal felony charges. He was sentenced to 17 1/2 years in prison and is housed at a low-security facility in Milan, Mich. Haugen, now 35, was convicted on four felony counts and received a five-year prison sentence. She is being housed at a federal facility in Texas. Gellerman was convicted of two misdemeanors and sentenced to three years of probation.

Minnesota has worked the past several years to pass laws controlling the sale of high-priced synthetic drugs with labels such as “bath salts” and “incense” that are analogues to street drugs including cocaine and heroin.