I know this might be an "unpopular opinion" type question, but have you considered what it's going to be like when you finish studying if your only method of concentrating and acquiring knowledge is based on pharmaceuticals? Like, what's the plan? Are you going to take stimulants for the rest of your life? Because any job you're going to use your qualifications for is also going to require you to concentrate for long periods and learn things.
I mean, there are people who do need to take stimulants for life, but if you're one of them you should probably talk to a doctor about getting an appropriate diagnosis and treatment that suits your condition. If not... then I hate to be a downer, but you should probably focus on trying to develop good study habits instead of looking for a medicinal solution.