The man was charged with selling the drug cyclopropylfentanyl online in the fall of 2017. Eleven of the buyers died after using the drugs.
He was sentenced to three years in prison by Linköping District Court for three cases of causing another person's death - the prosecutor wanted him to have been sentenced for eleven cases of manslaughter.
The prosecutor has appealed the verdict to the Court of Appeal.
The main trial of the Court of Appeal begins on May 10 and concerns, among other things, eight cases of murder.
The main hearing is expected to last for nine days.