Not trying to be funny - just getting your attention. This is an excellent but seldom discussed topic.
Constipation is a "side effect"' of having Parkinson's. We don't move as much, which really slows down the digestive tract. Also, many of us take anti-depressants, many of which cause constipation as a side effect.

I won't name them all, but most PD meds lend to diarrhea or constipation..

I know you mentioned diarrhea, but that can be a result of having constipation (Keep reading - it gets more confusing!) Many PD patients get "impacted" - sort of plugged up. This causes leakage or diarrhea around the impaction.

Impaction of the bowel can be lethal! In fact, it is one of the leading causes of PD death . Below are some excerpts from side effect listings of some commonly used PD drugs.

Straight from the rxlist.com website:
NOTE! The side effects listed does not mean you will definitely have them; this is just a list of potential side effects. you may look up these side effects or interactions yourself at the above website.

Side effects of:

Sinemet or carbidopa/levodopa: Gastrointestinal: dark saliva, gastrointestinal bleeding, development of duodenal ulcer, anorexia, vomiting, diarrhea, constipation, dyspepsia, dry mouth, taste alterations.

Requip/Mirapex: in order of decreasing incidence: nausea, dizziness, somnolence, headache, vomiting, syncope, fatigue, dyspepsia, viral infection, constipation, pain, increased sweating, asthenia, dependent/leg edema, orthostatic symptoms, abdominal pain, pharyngitis, confusion, hallucinations, urinary tract infections, and abnormal vision.
Comtan/
In clinical trials, diarrhea developed in 60 of 603 (10.0%) and 16 of 400 (4.0%) of patients treated with 200 mg Comtan and placebo, respectively. In patients treated with Comtan, diarrhea was generally mild to moderate in severity (8.6%) but was regarded as severe in 1.3%. . . . . Diarrhea generally resolved after discontinuation of Comtan. . Typically, diarrhea presents within 4 - 12 weeks after entacapone is started, but it may appear as early as the first week and as late as many months after the initiation of treatment. Diarrhea may be associated with weight loss, dehydration, and hypokalemia.

Post-marketing experience has shown that diarrhea may be a sign of drug-induced microscopic colitis, primarily lymphocytic colitis. . . .

If prolonged diarrhea is suspected to be related to Comtan, the drug should be discontinued and appropriate medical therapy considered. If the cause of prolonged diarrhea remains unclear or continues after stopping entacapone, then further diagnostic investigations including colonoscopy and biopsies should be considered.
These possible side effects also apply to Stalevo.

Artane: Potential side effects associated with the use of any atropine-like drugs, including Artane, include cognitive dysfunctions, including confusion and memory impairment; constipation, drowsiness, urinary hesitancy or retention, tachycardia, dilation of the pupil, increased intraocular pressure, choreiform movements, weakness, vomiting, and headache.

WHAT TO DO: The following is prescribed for PWP:

Exercise - just walk, twist your torso, stretches, anything - but don't be stagnant - movoe as much as you can.

Eat roughage (celer y, fiber, leafy veggies,, fruits like plums, apples, oranges.

Drink water every time you think about it.

Keeping your colon "clean" also helps prevent toxicity of the drugs we ara required to take.