Jack is constantly seen drinking and doing drugs.
Jack suffers from alcohol and drug addiction, and it's obvious from very early on in the film — he literally goes into the bar he meets Ally in because he's in need of a fix. He's often seen drinking straight from the bottle, taking pills from prescription bottles, and snorting drugs (he also smokes). He is at one point admitted into a rehab, and it's clear how deeply he struggles with the disease. Other characters in the film are also seen drinking, most often taking shots with Jack.