In handing down the guilty verdict, the district court in Varberg, western Sweden, also ordered Petzäll to pay 17,100 kronor ($1,860) in fines.
According to the court’s ruling, the 23-year-old Petzäll tested positive for three narcotics-classified substances, including morphine, for which he had no prescription when he underwent a drug test in January 2012.
The drug test also revealed Petzäll had two additional narcotics-classified drugs in his systems, but he was able to prove to the court he had a prescription for both.
Petzäll said he plans to appeal the ruling, which he labeled “idiotic”.
“I’m really pissed off, to be honest,” he told The Local.
“It’s ridiculous to punish people who are suffering from addiction.”
Petzäll added that he hoped to use his position in the Riksdag to draw attention to what he considered to be a stain on the Swedish criminal justice system.
“Drug crimes policy isn’t something that’s discussed much in Sweden,” he said.
“It’s horrible that drug addicts are stigmatized in the way they are. You can’t stereotype immigrants or homosexuals, but all drug users are lumped together into a group that deserves to be punished with fines or prison.”
In addition to the fines, Petzäll must also pay 1,325 kronor to cover the cost of the chemical analysis as well as 500 kronor to Sweden’s Crime Victims Fund (Brottsofferfonden).
Petzäll entered the Riksdag following the 2010 general elections, which resulted in the far-right Sweden Democrats gaining representation in the Swedish parliament for the first time.
But Petzäll announced last autumn he was leaving the Sweden Democrats to deal with a substance abuse problem, but would remain in parliament as an independent.
Less than a month later, Petzäll was involuntarily committed to a substance abuse treatment centre to prevent an overdose.
In February 2011, Petzäll spent a night in jail after a drunken outburst, prompting his first time-out from politics.