Joseph Gordon-Levitt Rips GQ -- Drugs Didn't Kill My Brother

7/18 -- GQ has released a statement saying, "The magazine stands by its reporting, the facts of which are fully supported—and have been confirmed in detail—by the Los Angeles County coroner's office."

Joseph Gordon-Levitt is lashing out at GQ ... claiming the mag was "irresponsible" and just plain wrong when it printed an article that said his older brother Dan died of a drug overdose in 2010.

JGL -- who happens to be this month's GQ coverboy -- posted a message on his website which reads:

"I’m writing this because I have a problem with what their article says about my brother. I’ll be honest, it really made me feel terrible. Here’s a quote:

‘…the elder Gordon-Levitt died of an alleged drug overdose in 2010. “It was an accident” is all Joe will say about that.’"

Gordon writes, "The 'allegations' to which [the writer] must be referring were made by a handful of gossip websites. They are factually incorrect according to the coroner’s office and the police department."

He adds, "I don’t like publicly speaking about my brother’s death, but I’m making an exception to correct this irresponsible claim."

"By the way, while I asked the writer not to dwell on how he died, I did say quite a bit about how he lived, and how much he means to me. Dan was a brightly positive, genuinely caring, and brilliantly inspiring person, and I liked the idea of such a wide readership learning about him. My parents and I are disappointed with what the article chose to focus on regarding this sensitive subject."