We are trying to figure out why my elderly grandparent has been hallucinating. She is a small woman and does not eat a lot. She takes a lot of vitamins and nopalea juice. She has been hallucinating and trying to find cause.

No, it is merely cactus juice and has no known side effects like this. Although it may not be healthy to drink too much. It can cause frequent bowel movements.

If she eats very little, perhaps a poor diet is the issue ? Also there are other medical reasons, that should be looked at, she needs to see her doctor for exam

Mental status change should be examined by a doctor. Please take her to the ER.