Originally Posted by mmk
Well, as those who have kids with ADHD can attest, there are drugs which can make you more calm and focused, e.g., adderall. ADD/ADHD drugs aren't necessarily illegal, but they are controlled substances. I don't know what the ITF rules are on these drugs, but in other sports they are legal if there is a prescription. However, I doubt that Fed is taking any such drug.
Those drugs are stimulants and are definitely on the list of banned substances for tennis players.