Only for neurotypical people. For other populations the effect is real, I don’t know about clinical diagnostic tests but layman identifications commonly use “self-medicating with stimulants” as a red flag for suggesting someone seek a professional opinion.

So “can improve concentration”? Yes. Clinically proven. “Can improve [anyone’s] concentration”? Well you’re right that’s not true. But that’s relatively recent settled science, in part because there are plenty of anecdotes going back to Coca Cola being the “Thinking man’s drink” with coca leaves in it. Cocaine makes some people more focused. As does amphetamine, and methylphenidate.

But we’ve fucked the words “can” and “may” in legal terms. There is no accident that RFCs use MUST and WILL for mandatory behaviors, or that they’re in all caps.