Philip Seymour Hoffman -- A Single Drink Ultimately Led to Death

Philip Seymour Hoffman's recent and fatal path down the road of addiction was triggered by an innocent drink ... sources close to the actor tell TMZ.

Hoffman had been sober for 23 years prior to shooting "The Master" -- but during a wrap party in 2012, the actor succumbed to temptation ... and celebrated the movie with a drink ... which quickly became a couple of drinks.

Hoffman acknowledged to confidants ... the drinks opened the floodgates. Addictions experts all say one drink can destroy an addict's life ... and that seems to be what happened here.

Sources tell us Hoffman didn't begin experimenting with heroin until 2013, after he had already fully fallen off the wagon. We're told he had long been curious about the drug.

It started with snorting -- and last May, he quickly checked into rehab because he was terrified it would escalate to IV use ... which it eventually did.