Dr. Liam Mac Gabhann:

Because an individual is neither single nor dual diagnosis. Each individual has a whole systemic set of complex needs. Even using diagnostic criteria itself is challenging and invalidates the experience of the person who is struggling with substance use and mental health and all of the other issues that come with that. It is actually unhelpful, other than politically and to try to get funding. Once it comes down to the person, however, it is quite unhelpful and probably insulting to reduce somebody to a single or a double diagnosis.