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It sometimes happens with dementia people. He should take Rivastigmine patch and dextromethorphan & quinidine (Nuedexta) if possible. If dextromethorphan & quinidine (Nuedexta) doesn't work, he should take low doses of olanzapine or risperidone. But they have limited efficacy and are associated with increased mortality in patients with dementia. If he can't see a neurologist soon, you should take him to a psychiatrist.
Yes, it is common. There are shots for the medications. But it could make his life shorter.