I have absolutely no real idea on this subject.
But the term " mental disease " is confusing to me.
Do you start with mental issues that lead you to look for a drug as a crutch ?
Or do you develop " mental disease " because you are on a drug ?
I believe it's both.
In the first instance, there is a massive connection between drug addicts and people with existing mental health issues. The statistics on this are pretty clear. So there is usually some form of 'self-medicating' going on as a lot of the time the people with these issues don't have access to or the knowledge of how to get help. The drugs change their mindset in someway and may temporarily relieve the mental health issues being experienced. This then forms a crutch which eventually turns into an addiction.
Additionally I have read of addictive personality types which may mean certain people are more susceptible to addiction. I'm not sure where this sits medically, in terms of being a mental health issue. I'm not sure I necessarily agree with it either but I have read about it. Remember that it's only a percentage of people that do use drugs end up becoming addicts so there must be someone else at play. I think last I saw it was around the 30-40% of Australians have used drugs and I don't believe there's that many drug addicts around.
On the second point, once addicted to a substance an addict's brain is essentially rewired which means they cannot control impulses and other normal brain functions. This rewiring is similar to how other mental illness work hence the assessment of drug addiction being classified as a mental health issue.
I'm not an expert on this by any means, only restating my understanding so if anyone has more in depth knowledge I'm happy to be corrected.