Excessive drug and alcohol use can readily cause "mental problems", that don't need to be described as two different factors as though he already had unusual mental issues earlier in life and drugs made such worse. Hard drugs alone are so overwhelming with what they cause in human bodies, that the rest tends to be of minor significance. That noted, it is true if some kid with actual mental problems uses strong drugs, that they are likely to get worse. But numbers of the latter I'd expect, are far far lower than what drugs alone cause.
Drugs use is an immense negative issue in this human science and technology era that I expect will become worse as designer drugs continue to arise. For some like those drinking caffeinated drinks, not an issue at all, and ought not be discouraged, much less banned. What one person with good self control and behavior can handle at some level of drug use effectively to make enjoyment of their life better, without impacting others, or damaging their own bodies, may not be so for the next person with poor self control and a warped mind.
There is no way for societies to set some clear, fair for all threshold on what is ok and what is not. Black and white prohibition has proven to be unworkable while open freedom of any drug use has likewise proven to make society worse, thus is unwise.