'Clerks' Star Lisa Spoonauer Overdosed on Painkillers for Cancer, Lung Disease

"Clerks" star Lisa Spoonauer died from an accidental overdose of a powerful painkiller ... TMZ has learned.

According to the Ocean County Medical Examiner in NJ, Lisa died of acute and chronic intravenous use of hydromorphone ... the generic brand of the narcotic, Dilaudid. She was also battling cancer, anemia, lung disease and immune deficiency ... all of which contributed to her death.

Lisa, who played Caitlin Bree in the cult classic film ... was only 44 when she died in May. At the time, her family said she'd been fighting undisclosed illnesses for years.

Her death prompted "Clerks" writer/director Kevin Smith to post an emotional tribute.