Tolkien does not write within the contemporary model of disease. He writes within the old model of moral failing.
I cannot recall any passage in The Silm which suggests any addiction in Melkor nor any reference to drugs or alcohol.
Tolkien writes within a moral universe, not a medical universe. Melkor succumbs to his own grandiose desires for power, control, domination, personal self-satisfaction, pride. Those terms are not found, if I recall correctly, in the
Wiki discussion of the Diagnostic and Statistical Manual of Mental Disorders, 4th ed. It might be tempting to think of him in these contemporary terms, but Tolkien's conceptual universe is very different.