The estimation by non-users that drug use produces a sudden, drastic and permanent brain deterioration in the users seems to have been unrealistically amplified by society, in my experience.
The estimation can cause non-users to discredit the assessments of users on general principle, which of course leaves their assessments the only valid ones remaining - for them, anyway. The article's question isn't likely to be resolvable within a context like that, because the typical result is just marked social division between users and non-users. I suppose the two social factions will just have to resign themselves to arguing the matter with no possible chance of resolution.
That estimation also produces other resultants, too: A societal justification for keeping most drugs on the black market, with all the private and government programs that drug money is used to fund. And for users, it keeps them reliant on a distribution network, at the prices they set, and limits both their quality assurance and selection of substances. Additionally, it should be noted that if you're a major drug distribution network with a lot of the say about what specific types of drugs become readily available within a country, you have the ability to partially shape the mindset, mood, energy level and attitude of a given generation.
The ability to influence that can be intensely useful for, say, politicians.
With all that potential incentive attached to it, that common estimation is beginning to seem less and less innocuous and naturally-occurring. When that estimation rubs off from "society", where - specifically - do people get it from? Ah, that's right. It's the slant on medical research data of prolonged, hard use of certain drugs, provided to us at an early age by government-controlled public schools and government-funded anti-drug campaigns in the media.
But it's not as if there could be a hidden agenda at work there.