Are controlled substances and illegal drugs the same thing?

No: A controlled substance is a chemical compound that is "controlled," per say, by the government. Access to these substances is limited, hence the use of prescriptions for potentially addictive pharmaceuticals.

An illegal drug is a substance deemed harmful, both to the body of one using said substance and those in the society around it, and its use is, therefore, not allowed under the law.

Well i do think they're the same.