I have read a lot about PD but am still unclear.
Is there a drug that slows the progress of PD or isn't there?
In theory, there is none. This means, there doesn't exist anything that was proven to slow down the disease progression. There is 1 thing, I found though ... a study about gou teng. Last year it was shown that over a period of 24 weeks, people taking gou teng slept better and dealt better with their symptoms than placebo controlled people.