My hypothesis is that much of it comes from need(s) being unmet consistently and traumas that can linger even when needs are getting met from times in the past that they weren't. Needs meaning basic things that include what people would usually think of like food, water, and shelter, but also includes less emphasized things (especially under capitalism) like intimate connection (can be platonic), sense of community (in the meaning of interdependence not fandoms), sense of purpose and direction, sense of safety and security.

Capitalism is far from the first or only system to fuck people up (formalized slavery comes to mind), but... if we're looking at it in detail, capitalism puts the working class in a precarious position where they are disposable, which fucks with any sense of safety and security; it destroys community; it warps relationships toward the transactional; it reduces purpose to "live to make money and buy product"; it leaves direction in the hands of liberal individualism. From the perspective of basic needs, these are a lot of very big problems and it's no small wonder people will turn to various unhealthy coping mechanisms, some of which plays right into capitalist profiteering; it has industries ready to fill the void with "treats" and the less fulfilling these treats are, the better (for the capitalist) because that means you'll need to come back for another hit more often.

Don't think it's only a capitalism thing, but I guess capitalism causes so much psychic damage both due to how individualised the burden of existing is and how you constantly have to deal with completely unnecessary nonsense. Also drugs are commodities, so they fit very naturally into "fixing yourself" in capitalism compared to other economic systems, despite all laws. My smoking buddies are usually either political activists or homeless people, and it makes sense as those are two sets of people who engage with the irrationality of the shape of society on a day to day basis.

Its a function of neuro-chemistry not society or capitalism. People have been getting shitfaced since before the written word. Animals get fucked up on stuff if given the chance.

depression is certainly a side effect of capitalism. obviously there are other things that can cause it but more people are depressed than ever before (more or less) and i think the social isolation encouraged by current-stage capitalism is a factor. a lot of people choose to self-medicate. there have been times in my life i would get really down every month or so and decide to get really fucked up about it. usually it made me feel a bit better.