Earlier quoted context omitted.
Yes. Some brains react to some stimulants differently. That's the whole reason ADHD can be treated with Adderall.
This isn't true, it affects everyone the same way. But you could say the optimal dosage is different for different people. I think people just like saying this because they're afraid stimulants will get banned otherwise.
That's why just having them illegal makes them 100x more dangerous. Through less knowledge among users, no guidance on packaging and difficult to identify the substance if someone had to be taken to the hospital.