I've seen depictions in passing in film (Apocalypse Now, American Gangster...) but how was it in reality? Would soldiers overdose? Would they use in combat, or to combat the constant stress of war? Are there any documentaries or other films/novels that show this side of the war?
Well.
Specifics would help. Which drugs? Getting even more specific, which soldiers, where? Air force, marines, navy, and army all conducted themselves differently, and saw different sides of the war. The US was issuing soldiers stimulants, they were using recreational drugs to help deal with the stress of combat, and when you get right down to it alcohol, tobacco and, yes, even coffee, all could constitute drug use.
When you're lumping stimulants in with depressants, you start producing patterns of behavior that are at odds with each other.
If you're talking about marijuana or narcotics like heroin, then the drugs had a massive negative effect on American combat effectiveness and discipline.
By 1969, the U.S. Army was rapidly deteriorating and had operational paralysis. There were several factors for this, but drug abuse was one of the major ones. It contributed to massive indiscipline, "fragging" of officers, and combat refusals.
A medic at a military hospital in Binh Thuy, recalled that "heroin usage was rampant." "'These guys would work stoned all the time. My medics would shoot up patients.... I caught them in the bathroom shooting each other up.'"^1
Another one recalled "'In my units the majority of people were high all the time.'"^2
Here are some excerpts from The Generals by Thomas E. Ricks:
the U.S. Army was a mess. In October 1970, it was losing a soldier a day to death by drug overdose.^3
Rear areas were in shambles, with racial tensions high, discipline becoming optional, and marijuana and heroin widely available at rock-bottom prices.^4
in 1965 the Army in Vietnam had 47 "apprehensions" for drug violations while in 1970 it had more than 11,000^5
Perhaps the best example of how detrimental drug abuse was to the US Army, was the Massacre at Firebase Mary Ann. In the middle of the night on March 28, 1971, NVA soldiers were able to infiltrate the perimeter of the Firebase because routinely soldiers got high on weed and either fell asleep on guard, or didn't even report for duty. The NVA wreaked havoc on the base and killed 30 Americans and wounded another 82.^6
As a side note, this continued for a few years after the Vietnam War. The US Army basically disintegrated into being near worthless by the mid-1970s.^7
Endnotes:
Edit: fixed some proofreading errors.
I found a nice journal article that discusses drug use of returning soldiers U.S. Army Enlisted Men in 1971-2. From the journal it seems like it was fairly common.
"Almost half of the “general” sample tried heroin or opium while in Vietnam and one-fifth developed physical or psychological dependence."