The question is a good one.

Soccertese is correct that the FDA doesn't recognize any drug as slowing the progression of PD, but it is worth making the point that that is different from saying that no drugs or other types of therapy actually do slow progression in some people.

For FDA recognition to be awarded stringent statistical claims need to be justified by the data. Trials to collect this data cost a lot, and have a good chance of ending in commercial failure, in the sense of not winning FDA approval. This could happen even though the data is slightly positive across the cohort on average or strongly positive for some people. (There are probably many types of PD.)

An observation that I make is that some people progress more slowly than others. This could be because of the nature of their specific form of the disease or what they do to try to slow its progression or both.

So, my attitude is that it is worth each one of us looking for what works for us individually.

John