What causes delusions?

Brain cell death; dementia; alzheimers.

Things like Brain cell death, alzheimers, and also dementia which is a chronic or persistent Disorder of the mental processes cause by things such as injures or brain disease.

A hallucinogen. Most are dangerous. People abuse these to escape from what they perceive to be a painful reality, rationally or not.

My sister had surgery to form a colostomy yesterday. She is now having delusions, and hallucinations. She is also abusive. Could a medication have been a cause of this?

Delusions-- A false belief regarding the self or persons or objects outside the self that persists despite the facts.