From my experience as an undergraduate, there are a number of drugs that can be used to enhance your ability to focus. These are typically extended release amphetamines i.e. Adderall, which they give to people with ADHD, and Nootropics.

Adderall is common because of the ease of getting a prescription in the US for it. All you need to do is tell a doctor you have trouble focusing and they will probably give it to you. Which is sad to me, as they get kids put on this while they are still in middle school and it definitely effects your behavior (you become more pacified and focused from my experience). A lot of students use Adderall because of its ability to help them focus on the topic, and work for extended periods of time (i.e. all night and day) to wrap up that research paper they should have started weeks ago. I've read that learning under the influence of Adderall makes it harder to recall that information when you are off the drug, so I never used it in class. Only for the use case I mentioned above, where I needed to work on sometime for an entire weekend straight to turn it in on time. That being said, I'm probably in the minority when it comes to that... I think it is heavily abused.

Nootropics on the other hand are probably closer to the subject matter of your post, as they are referred to as "smart drugs". There are some nootropics that you can buy legally, but from my experience they did not do anything (I tried them for about a month). Apparently the good stuff is Modafinil, but I have never been interested in getting ahold of it to try. Personally, I do not like the idea of using drugs as a crutch for my cognitive ability. That's why I only used Adderall to stay awake yet focused, instead of all the time for learning.