https://pubmed.ncbi.nlm.nih.gov/26273459/
“In this single-center, double-blind, placebo-controlled, randomized clinical trial comprising patients with early- and late-stage PD, supplementation of a single daily dose of niacin for twelve months improved the rigidity, bradykinesia, and overall UPDRS III scores. High-dose niacin supplement was observed to reduce bradykinesia and rigidity”