What Causes Delusional Thoughts In A Person?

dear doctor, warm greetings! one of my friend apparently normal before 2 weeks back, became drowsy for 2-3 days followed by thinking too much about past and talking about past events now, saying some people might have done this and became suspicious about people, media and even food. For me this sounds more like delusional rather than true facts. I would greatly appreciate if any drugs that may cause this type of behavior ,ie remembering the past events and getting suspicious. Thank you.