In an indictment filed on Monday in Varberg District court, prosecutors charged Petzäll, 23, with minor drugs offences after results of a January urine test showed traces of several narcotics-classified substances for which he had no prescription.
Among the drugs revealed by the test to be in Petzäll's system were morphine, the anti-anxiety drug Oxazepam and Clonazepam, a muscle relaxant, all of which are classified as narcotics in Sweden.
After the indictment was made public on Wednesday, Petzäll used his Twitter account to comment on the charges.
"Charged with narcotics crimes despite a certificate that I have legal access to what I tested positive for," he wrote.
Petzäll had previously denied the charges in an interview with prosecutors, according to court documents.
Writing further on Twitter, he claimed he had provided police with the necessary documentation to clear him of the charges, which he learned about through Swedish media reports.
He went on to criticize police, calling into question their motives in handling his case.
"But of course, it's no secret that the police like to mess with people and do everything they can to doctor the statistics," he wrote.
"Cops have and shouldn't have anything to do with addiction. It's a question for the healthcare system."
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.
Last September, however, Petzäll announced was leaving the Sweden Democrats as he struggled to deal with a substance abuse problem, but planned to remain in parliament as an independent.
At the time, he said he wanted to continue to use his seat in the Riksdag to "push my pet issue and increase awareness. To develop more concrete measures to help young alcoholics and drug addicts."
Less than a month later, however, Petzäll was involuntarily committed to a substance abuse treatment centre to prevent an overdose after he was taken to a Gothenburg-area hospital by ambulance in late October.
According to Aftonbladet, the 23-year-old MP has struggled with addiction to Valium and methadone, a drug often used to treat heroin addicts.
Petzäll first took a time-out from his political activities in February 2011 after a drunken outburst in Trelleborg in southern Sweden resulted in him spending the night in jail to sober up.
The party was being hosted by his ex-girlfriend and according to a police report Petzäll caused damage in his drunken state.