Slurring/Memory

My father was diagnosed just about 6 months ago with 6-12 months to live, pancreatic cancer which has metastasized, especially to the lung (or from, who knows).

He was okay until about 3 weeks ago and now is slurring his speech, losing memory. Is it the drugs? Has anyone else had this experience? He is using morphine and has been for about 2 months. But is that the cause? He knows who I am but forgets where I am living and veers off on a tangent to the actual conversation.