No big secret: in the 1960s, lots of people were doing drugs. LSD was the hallucinogen of choice, easily available and affordable. There are lots of old people (hey—who you calling old?) around who still have acid (LSD) flashbacks when they're standing in front of a Red Box or buying paper towels at CVS. You can't buy your Snickers until it's over. It's not a good thing.
Hallucinogenic drugs will mess you up, mmmkay?
Surfer-soldier Lance Johnson hasn't learned these lessons yet. In fact, he drops acid just as they reach the last U.S. outpost on the river.
LANCE: Hey you know that last tab of acid I was saving. I dropped it.
CHEF: You dropped acid? ...Far out.
This doesn't get him killed as they're walking around, but it makes the war seem like some sort of strobe-light, fireworks freak-out. Like being at an Avicii show. The whole film has that dreamy, trippy quality, no LSD needed.
You could argue that the movie is saying that the Vietnam War itself is like a national acid trip—a bad hallucination. The soldiers encounter incidents that feel almost hallucinatory in their strangeness—surfing with Kilgore, the tiger, the Playmates, the last outpost, and finally Kurtz. Unfortunately, it's all real. There's no way out of this bad trip.