I wouldn't take Mannitol if it didn't have such a massive benefit for me.
Here is another person on healthunlocked/ parkinson's movment.
"I started Mannitol about 3 weeks ago with some pretty good results! First of all, I have been able to leave my daily Mirilax dose in the cupboard. (After YEARS of taking it daily just to keep my gut moving. If it did nothing more than that, I would still be thrilled.) Next, I noticed that my energy is vastly improved. Don't seem so tired all the time. My cognition is a bit sharper. My voice is noticeably stronger and clearer. And my fine motor skills are improving. I'm still taking Sinemet but have been able to cut the dose in half. Still also taking Amantadine and Epsilon patch. My whole attitude is improving. I am taking one teaspoon daily. ( I weigh 140 pounds.) I did try various doses and discovered that too much actually had me more jumpy/tremmory. Hope that long term use brings even more improvements! Feel like there's hope for the first time in years!"
Again to reiterate it doesn't work for many, but a significant minority do report great benefits.
Given the iron issue, perhaps don't take it with high iron foods (as it doesn't last very long in the body).