I'm Just Curious: Why Is It OK For A Person

I'm just curious: Why is it OK for a person diagnosed with ADHD to take Ritalin and normals not? What would happen if "normal" took same dosage of Ritalin (or similar drug) in same way as "ADHD" person? Is there any danger in that? Surely, since ADHD not diagnosed with physical symptoms but with observation and anecdotes, a good percentage of those so labeled are labeled wrong. So, they take the Ritalin and...what happens?