Souvenaid is a drink supplement that has shown to improve memory function in placebo controlled clinical trials for mild Alziemher's. The latest research in this area was published in 2015, where 201 participants took Souvenaid or a placebo for 48 weeks.
At this time we haven't been able to find any evidence it has been tested in people with Parkinson's. So we do not know if this supplement would have the same effect in Parkinson's.
I hope that helps answer your question.
All the best,
Parkinson's UK Research Team