There is now an informal "trial" of mannitol for PD, being run via a website in Israel. Anyone with PD can take part. The "trial" has been running for about 3 months and they have about 800 participants so far.
They have structured the website such that you get to see the dosage recommendations after you complete an initial survey.
I joined about 3 weeks ago. I now add 1 level tablespoon of mannitol to my morning coffee, and fill out an online survey once a month.
At this stage I plan to hang in there for at least 6 months, maybe a year (the original researchers gave mannitol to fruit flies for about a month before retesting them, and it was 4 months for the mice).
JustCloud-IT download page
(hat tip to Ranico at HU for the link)