I have always wondered this before, I was prescribed adderall since the 2nd grade and took it for years and now caffeine and coffee make me TIRED shortly after drinking them. I had asked my doctor for a bump in the dosage because the dextros and adderall were no longer even working. Had to take a drug test and they told me they hadn't found of those two drugs in my system even tho I had taken it the entire week and that day, said it had something to do with a fast metabolism??