Is it possible to have both Parkinson's & Alzheimer's or is it more likely to be Parkinson's dementia?

It's possible to haved mixed vascular (with or without Parkinsonism) and Alzheimer dementia, but vascular is more common and would explain both things, so I'd tend to go with that. MCI (mild cognitive impairment) is not specific to either one at early stages. Some people still use "Alzheimer's" as a generic term for all dementia - you even see some medical journal articles that mix them together, often inappropriately. As far as any given drug helping, you pretty much just have to try it and observe over a reasonable amount of time. Aricept may help some in this situation. My mom had predominantly vascular dementia and Parkinsonism, and that plus most of the other things we tried just made her more cranky, more delusional, and more likely to take a swing at someone with her purse, except for Sinemet, which was a huge help in terms of her being able to feed herself, etc. which gave her a lot better quality of life.) In general, people with what you are describing should avoid antipsychotic medication if at all possible, though it is not absolutely ruled out given symptoms that justify it and respond to it. (If she's got Lewy body dementia and ever has a few of the hallucinations typical for that, that alone would probalby NOT justify it, especially if they are not that disturbing to her and/or she knows what they are!) Sorry you are starting down this road - hugs, and hopes and prayers for at least as much good quality time as well as heartache!

We had noticed forgetfulness in Mom before, but nothing earth shattering. She had even had a couple of mini-cognitive tests, which she passed. But the past month, it's started going downhill rapidly.

She just now came in and told me that she was going to take her Aricept (we agreed today that she was going to notify me when she was taking pills, because she has been forgetting to take some). I told her that she had already taken it today, and she said, "Oh, I'm supposed to take it 3x/day". I told her that was her Parkinson's medication, not the Aricept, so we went back together to check (she didn't believe me) and found out that she was about to overdose herself. I hate that this is starting.

Thank you for the hugs, hopes and prayers. I think I'm going to need them.

Yeah, she needs the supervision now for sure. My mom seriously harmed both herself and my dad with medication errors before any of us realized she could not read the new list for him after a hospital discharge, and kept giving the one she was supposed to stop, and then finally overdosed herself on her statin which is probably what led to her taking a fall at home and losing her independence for good.

You are doing it right, to let her do what she can for herself, but supervise, supervise, supervise....more hugs and prayers headed your way.

Yes, both. A lady in my Alzheimer's support group has a dad w/ Parkinson's who developed Alz about 2 years later. He is/was on Aricept, some improvement, but she said it was hard to gauge, as the 2 diseases basically took turns developing new effects & behaviors.