“Brain fog” and “boost your memory” are among the most searched health phrases on the internet, and among the most heavily monetised. If brain fog is persistent or getting worse, the right first step is a doctor, for reasons that will become clear. Because the useful thing to understand is that brain fog is not a diagnosis. The question “how do I fix brain fog” has no general answer, because the fog is a signal, not a thing in itself. But the most studied memory supplement, given the fairest possible test, did nothing.