Since human beings have encounted stimulants, we have been fascinated by the potential effects of substances on cognitive function, memory, and intelligence, as,for example, exemplified in the theme of the film "Limitless". Currently, brain imaging is being used to market supplements that supposedly enhance brain function, memory, and, by implication, intelligence.
This is the essence of positivism in a reductionist approach to scientific inquiry. Accordingly, increased brain activity is equivalent to more or better use of the brain somehow and that the absence of activity is equivalent to less effective brain function.
Aside from the evidence indicating that the brain can metabolize glucose anaerobicly, as do muscles, who is to say that the surface of the brain does not act more like the retina in terms of integrating in transmitting the signals from impinging light frequencies to the visual cortex: signalling to the brain from the retina is based on inhibition of intrinsic activity. In essence what we see is sculpted by what gets turned off. This was suggested by Pribram in the early 70s as a mechanism by which information enters consciousness.
Taking a drug to general increase activity in the brain in a non-specific way has nothing to do with intelligence, and while overall capacity might increase in a transient way, while a drug is active, history has shown us that there is invariably a downside to using stimulants as cognitive enhancers.