I've had concentration problems the entire 2 years I've had PCS. I always feel foggy-headed and my working memory is bad so it impairs my focus if I am presented with too much information at once. My brain gets "tired" more easily. While on adderall, my brain doesn't get as tired and I feel I have more cognitive ability to use all the brain power I have. Before I had these most recent concussion problems, I could take an instant release adderall or two and have no feeling of comedown other than slight tiredness. While having PCS this effect was greatly amplified, and is now still kinda bad. Also with PCS now if I have too much I might even have a bit of depression on the comedown. I think how negative an effect it will have will depend on how bad and what kind of symptoms you're having with your PCS. And about coffee, I'm not exactly the same as I was before as the mental energy effect it can provide is not nearly as strong for me now and I feel I have become more tolerant to the positive effects and more sensitive to the negative effects. But slowly, all of this is improving.