In the Land of Metal, there exists a tribe that has upheld the gospel of Manowar with such zeal that it modified their bodies.
Metalheads descend from humans, and may look human at a glance. A more careful examination, however, will reveal the traits their kind has acquired through the ages. These traits allow them to have a lifestyle that would be extremely harmful for humans; the man who tries to live like a metalhead usually does not live long.
For a regular human, a mix of cannabis, cocaine, morphine, sleeping pills, cough syrup, LSD, Rohypnol and alcohol might be a suicide cocktail. For the metalhead, however, that would sound like recreation to last a weekend. Tolerance to those substances does not decrease with age.
Case study:
Since metalheads have descended from humans, what adaptations could the human body develop to cope with elevated serum concentration of the drugs mentioned above without any lasting harm?