The Vietnam war was the cause of several things today, none as apparent as the drug crisis. We had millions of young men and boys in a very venerable state not knowing if today was their last. They were turned into heartless killing machines watch kids burn to death with napalm on the regular. You can't destroy a person’s heart without lasting effects, these lasting effects were depression, ptsd, anxiety and other disorders which made them more susceptible to substance abuse. The Vietnam war brought millions of addicts back home without a support system to help, them spreading the pain of addiction. 
In Vietnam is started with Marijuana and was consumed by most of the military and was used to help cope with their day-to-day struggles. Marijuana was the main substance for a while, but the government started catching people and doing large burns of the plant. Soldiers were being punished for smoking marijuana and since it has a strong odor with no way to get rid of it the soldiers adapted. They adapted by using other substances, far more dangerous and far more addictive.  
One sort of drug soldiers used was psychedelics in particular, LSD and magic mushrooms. These drugs make a soldier feel more “enlightened” and at “peace”, they are hallucinogenic that can make people reflect on things usually trying to alter “negatives” they see in themselves. It’s not real change but with constant use and abuse of these drugs it can cause horrific permanent psychosis and brain damage. These helped a lot of men find peace when there was none but still was ripping them apart in another way. 
The last and most dangerous drug was heroin, the most deadly and active drug on the planet. They consumed it by moving the tobacco out of the end of their cigarettes and replacing it with a white odorless heroin pellet. During this time “51% of the armed forces smoked marijuana, 31% regularly used psychedelics and 28% were addicted to heroin”(Janos). With such high numbers of addiction across 2.7 million soldiers we had 750,000 heroin addicts. Heroin isn’t something you can just kick it is the most wonderful and joyous feeling on planet earth. These guys were hooked for life and couldn’t get enough. Unfortunately, The US government yet again failed its soldiers by not giving them any support when they returned. We left 750,000 heroin addicted, ptsd filled addicts to fend for themselves with no rehabs, supports groups or anywhere they could go. Just like with all drugs heroin spread like a wildfire once it had the volume of demand to create reliable markets. Cartels were born and once peaceful small towns were filled with junkies in a couple years.   
This created normalcy around hard drugs which led to cocaine being widely accepted and used by many and it has just been a slow and never-ending spiral. Now cities are overrun by the homeless who are addicted to the exact same substance our soldiers were several years ago. This is how the Vietnam war contributed to addiction in America.