My dad a stroke about 3 years ago. He had never been in a hospital before at 81 years old. They gave him something I believe it was morphine to calm him down because he refused treatment. He started hallucinating within 30 minutes. The doctors was saying he had dementia. We took him home and it took about 3 days for the affects of the morphine to wear off. And for about two years he didn't hallucinate, he just had short term memory loss. Then four weeks ago he fell. He broke his shoulder blade and a rib at home. We took him to the hospital and they gave him morphine and the hallucinations were back. I swore that the morphine was making him hallucinate but the doctors said it was dementia. We took him home and hospice kept upping the dose of the morphine. Finally he got so bad we couldn't take care of him at home. He went to the hospital and finally a nursing facility for 11 days. He died there after his body shutdown. He wouldn't eat or drink anything. He knew all of his family until he died last week. I will always believe that the morphine that was used to make him "comfortable" helped kill him. This is the same process my grandmother went through in 1987 when she died. They've been using morphine for over a hundred years. It's a killing agent and no one can convince me otherwise.
Meg, I can't help, but I have had a similar experience with my mum. In 2002 she had a very bad fall in the middle of the night and was taken into hospital. She was in considerable pain, but lucid and coherent. She was put on Morphine for around 2 days. During that time she suffered severe hallucinations, and has never been the same since. Every time she has a shock (like my father dying, or minor cataract surgery) she suddebly gets worse, but seems to plateau the rest of the time. She still has not a diagnosis but I suspect dementia. As you say, it does seem that it is either there and something activates it or it can come on suddenly. I am at the end of my tether with it, and just wish I could get a diagnosis.