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 revreation to last a weekend. Tolerance to those substances does not decrease with age.
Since metalheads have descended from humans, what adaptationd could the human body develop to cope with elevated serum concentration of the drugs mentioned above without any lasting harm?