In too many cases, there's a direct line between PTSD and addiction. In American pop culture, PTSD is directly associated with the military. That's not surprising, since most people first or have only heard the term in association with the Iraq War (pick one) or the U.S. occupation of Afghanistan, so it's inevitable that the story of a junkie veteran would come back to the fore, war after war.
In the case of
Cherry, adapted from the confessional and semi-autobiographical novel by Nico Walker, the titular Cherry (Holland) makes one key, pivotal, dumb decision. When his girlfriend, Emily (Bravo), dumps him to go to college in Canada, in a fit of pique he signs up for the military as a medic. His deployment to Iraq in the mid-2000s (a pointless mix of not being able to help the locals and scooping the guts into eviscerated grunts he'll never see again) leaves him with chronic PTSD. He doesn't develop his habit until he gets back, but a combination of chronic nightmares, prescription pills, and friends with a little extra lay all the stepping stones towards his eventual fate: holding up banks to pay for his next fix, and getting deeper and deeper into addiction and trouble.