Obviously in this case I’m not talking about profoundly retarded people but those who are closer to the normal end of the scale.
Google, “mentally retarded drug abuse rate”.
“Drug Abuse among Mentally Retarded People: An Overlooked Problem.”
Here’s a more recent review
A review of substance use research among those with mental retardation
If you are already retarded, what possible problem would drug or alchohol abuse pose?
Same as anyone else: interfere with your ability to hold a job, lead to health consequences, squander scarce resources, interfere with your personal relationships.
Specific to the mentally retarded, getting involved in illicit activities could be especially detrimental to people that don’t understand the consequences/risks they are taking and can be more easily led astray by bad company.
The rate of drug abuse among prisoners is high, well over 60% and given that around 70% are inside for drug related crime, the true figure is probably much higher.
It’s also true that 80% of all convicted male prisoners have at least one or more personality disorders, that 90% have a low level mental problem, and you would likely conclude that at the intersection of crime, drugs and mental illness there is a correlation. You will find as you scour the net that figures vary from around 70% upwards to above 90%.
That is not to say any one is causative of the others, but taking hard drugs during the teens, especially early teens may well have an effect on mental health.