What is difference between dose and fix? Are these words interchangeable or not?
I assume you are talking in terms of drugs.
A dose is typically a more medical term. It refers to a set amount of a certain substance.
A fix is a slang term. It basically means whatever you need to fulfill the drug craving.
Dose and fix would be interchangeable if you were talking informally, but you would never use the word 'fix' in a formal setting.