Ok, I just did a bit of searching for studies on hunter-gatherer use of drugs.
This paper [1] supports my characterisation of being mostly for shamanistic/ritualistic use.
And this paper [2] suggests that drug use also had medicinal benefits. But that is from a modern hunter-gatherer society that has undoubtedly been influenced by outside societies (indeed the drug they smoke has been imported from another continent), so I'm not sure how indicative of pre-historic hunter-gatherer use it is.
Where I remain dubious is the assertion that drugs have always been used for 'stress relief'. My feeling is that stress - as we understand it - was not something much experienced by hunter-gatherer societies.
Can we really say this? Are we sure that finding a patch of 'those' mushrooms wouldn't have just meant party time?
We know that (for instance) animals seem to seek them out...