Stephan Bonnar’s Cause of Death Revealed, Fentanyl Behind UFC Star’s Passing

Stephan Bonnar
Tito Ortiz and Stephan Bonnar Credit: Dave Mandel/

An accidental overdose of fentanyl and other toxic substances led to the untimely passing of UFC Hall of Famer Stephan Bonnar last December.

Bonnar’s death at age 45 shook the MMA community and led the UFC, a promotion “The American Psycho” has been widely credited with helping save, to host a Celebration of Life in honor of the fallen fighter.

At the time, promotion officials declared that the retired star had suffered heart complications. However, according to a new report by MMA Fighting, autopsy results revealed that “Fentanyl, Parafluorofentanyl and Mitragynine intoxication” led to Bonnar’s death on December 22, 2022.

Fentanyl, a synthetic opioid, is believed to be responsible for tens of thousands of overdose deaths in the United States alone since the late 2010s, and is often mixed in (unbeknownst to users) in street drugs to increase potency.

Parafluorofentanyl is a structural analogue (basically, a similar compound) to fentanyl. Mitragynine has been used for its psychedelic properties, but is also the active ingredient in kratom, a painkiller that has been deemed unsafe and ineffective in clinical trials, but that can be purchased in supplement shops.

Bonnar shot to fame as part of The Ultimate Fighter in 2005 at a time when the UFC was dangerously close to going under. His fight at the finale with Forrest Griffin has been credited by UFC President Dana White as helping to save the company, and while Griffin won the bout, both men were signed to UFC contracts.

The pair later entered the Hall of Fame together, entering the Fight Wing as part of the Class of 2013.

Bonnar wrapped up his fighting career with a loss against Tito Ortiz in 2014. In 2018, he was hit with a DUI; in 2021, Bonnar posted footage of an incident that saw him confront hospital staff and police at Henderson Hospital in Nevada as he attempted to acquire pain pills. At the time, Bonnar stated he had a fractured vertebrae stemming from pro wrestling training.