In an adult brain, defined as a brain that's 24 years or older, there are really only three things that are likely to cause a profound personality change in someone. These are:
- substance abuse or chemical dependency
- mental illness
- physical ailment, such as a brain tumor
Mind you, this isn't the same as watching a person get a little down in the dumps, or go through a period of anxiety. Profound personality changes are much more extreme. If you witness this, it's advised to reach out to a professional for guidance and counseling on how to address your loved one.