The substance was not specifically named by the police but Bild says an "eyewitness" claims Strassmann was out with three Slovenians that night and the parties had ingested magic mushrooms.
At around 10 pm, Strassmann reportedly left the group quickly and unexpectedly leaving his cell phone behind. He never returned.
After being reported missing and a lengthy search that took over a week, Strassmann's body was discovered in the Llubljanica River.
In Slovenia Visiting Friends
The 29-year-old Strassmann, who was one of the most successful high-stakes poker players in the world, was in the Slovenian capital of Llubljana to visit friends and do some poker coaching.
His disappearance created an outpouring of concern from the poker community and a dedicated search effort running 24 hours a day, spurred on by a Facebook support group.
No foul play is suspected in his death, and the autopsy confirmed violent crime could be excluded.
Find the full text of the story including the police spokesperson's statement (in German) here.