About 50% of Parkinson's patients have diabetes or un-diagnosed diabetes. Diabetic patients are twice as likely to get PD, but using metformin alone reduced risk of PD as if the patients did not have diabetes:
ncbi.nlm.nih.gov/pubmed/224...
Reduces weight and cholesterol:
diabeteswellness.org/metfor...
Someone's trying to get a clinical trial started:
cureparkinsons.org.uk/metfo...
Metformin has been used "forever" in diabetics and has been studied in life extension experiments in animals. It is taken by mouth, reaches the brain in at least animals, is a small enough molecule that it has a good chance of entering the human brain (MW=129, well below the ~400 limit)
There are 18 articles in pubmed related to PD and metformin. 11 of them are in the last 2 years, so there is a rapid increasing interest:
Here is a summary of the above 18 articles:
"Metformin is widely used to treat type II diabetes and other metabolic syndromes. In addition, it has been shown to increase neurogenesis, spatial memory formation and reduce the risk of Parkinson's disease....These findings suggest that metformin protects against haloperidol-induced catalepsy through inhibition of oxidative/nitrosative stress and has the potential for adjuvant action in the management of Parkinson's disease."
ncbi.nlm.nih.gov/pubmed/245...
Metformin restores AMPK (whatever that means) to possibly benefit PD, twice in animals models:
ncbi.nlm.nih.gov/pubmed/260...
ncbi.nlm.nih.gov/pubmed/261...
ncbi.nlm.nih.gov/pubmed/248... (aspirin also an AMPK helper)
Metformin blocks a-syn misfolding:
ncbi.nlm.nih.gov/pubmed/248...
ncbi.nlm.nih.gov/pubmed/263...
ncbi.nlm.nih.gov/pubmed/242...
"Metformin a well known antidiabetic drug has been recently investigated and proposed to promote neurogenesis and enhance the spatial memory formation.... long-term metformin treatment resulted in significant improvement of the locomotor and muscular activities in MPTP-treated mice than acute treatment. Metformin treatment significantly improved the antioxidant activity as compared to MPTP-treated group. ..TH-positive cells ... were found to be protected from degeneration in metformin-treated mice (47%, P<0.01). Interestingly, BDNF levels were found to be significantly elevated in metformin treatment group.... In conclusion, metformin possesses neuroprotective activity and provides preclinical support for therapeutic prospective of this compound in the treatment of PD."
ncbi.nlm.nih.gov/pubmed/251...
" metformin might exert a neuroprotective effect on PD via the restoration of parkin."
ncbi.nlm.nih.gov/pubmed/257...
But it reduces vitamin B12 status
ncbi.nlm.nih.gov/pubmed/222...
Here's a paper that indicates a possible connection between PD, diabetes, and AD:
"similar to the role of Ca(2+) ... in the pancreas of diabetic patients and the "Ca(2+) bridge" in ..
α-synuclein in the Parkinson's disease, ... the influence of Ca(2+) ... leads to
neuronal membrane damage in AD."
ncbi.nlm.nih.gov/pubmed/264...
"there are already randomized trials evaluating several established treatments for insulin resistance (pioglitazone and exenatide) as possible disease modifying drugs in Parkinson's disease, "
Modeling idiopathic PD as a complex illness - infections
Research finds Taurine to be neuroprotective in a Parkinson's disease mouse model 
