Drummer Duke Gadd's Cause of Death Revealed

Duke Gadd's cause of death has been released ... the drummer died of an overdose of fentanyl and cocaine -- mirroring what Corey Feldman originally said after he passed.

According to a spokesperson for Clark County, Nevada ... Gadd died from "combined toxic effects of fentanyl and cocaine." He also suffered from high blood pressure.

Remember ... Corey announced the death on social media -- saying Duke died in Las Vegas and sharing a short clip of the musician banging away on the drums.

At the time, Corey said it was a fentanyl overdose ... but, Gadd's family fired off at the star -- with his mom, Carol Gadd, saying they hadn't received final word from the coroner yet and sarcastically calling Feldman "Dr. Corey."

Initially, Carol said they'd heard Corey had a heart attack -- and a drug was probably involved, but no one specifically said fentanyl.

Worth noting ... Corey ultimately took down the Instagram post about Duke -- so, it seems he realized he waded into territory he probably shouldn't have.

Duke was the son of legendary drummer Steve Gadd. He was 41.