if someone takes drugs and suffer illusions. Is it our brains setting that illusion or is it drug itself?

Question: if someone takes drugs and suffer illusions. Is it our brains setting that illusion or is it drug itself?
- Keywords:
  brain,
  drugs
Asked by coolscientist to Ed, Hayley, Jason, Nathan, Sophie on 18 Mar 2013.
- Hayley Evers-King answered on 18 Mar 2013:
  
  It’s the effect of the drug on the brain. Some drugs have common effects i.e. they make you tired/sleepy/happy/sad but hallucinogenic drugs can have lots of different effects depending on whats already happening in your brain.
- Nathan Green answered on 18 Mar 2013:
  
  The brain reacts to the drug. Timothy Leary said its to do with “set” and “setting” which means how youre feeling and where you are. If youre feeling nervous in a loud room and drink lots of coffee youre going to feel much worse!

Question: if someone takes drugs and suffer illusions. Is it our brains setting that illusion or is it drug itself?