Drugs
It's hardly surprising that a clandestine market for medicines existed throughout conflict because there has always been one. The extent to which this was prevalent throughout the Vietnam War is unexpected. The number of drugs used by US soldiers was quite astounding, and the black market wasn't in the United States; it was abroad.
Over 51% of soldiers reportedly used marijuana, according to government reports. Although not a startling statistic for most citizens today, the report's introduction in 1971 would have raised concerns. The 31% who had used psychedelics like LSD or mushrooms and the 28% who had used heroin or cocaine were more worrying. It might be problematic if a quarter of your fighting squadrons are armed with machine guns and high on heroin.
In addition to the totally authorized drug use that saw soldiers being pumped full of speed, painkillers, and whatever else command believed they needed to keep going, there was also this illegal drug use. It's somewhat amazing that anyone survived to leave that nation.
The drug was affordable and widely available for American soldiers. An average container cost $5. It might be exchanged for a pack of smokes. Many soldiers resorted to heroin when the Army cracked down on marijuana use in response to a domestic outrage, since it was equally accessible and didn't smell bad. The military didn't regret their choice until after the fact.