Can cannabis help Parkinson's disease symptoms?

Parkinson's disease (PD) involves progressive dopaminergic neuron loss in the substantia nigra — a region with high CB1 receptor density. The ECS modulates dopamine release in the basal ganglia, providing biological rationale. Evidence by symptom: Motor symptoms (tremor, rigidity, bradykinesia): a 2014 survey (Lotan et al.) found significant improvements in tremor, rigidity, and pain after cannabis use. A 2019 RCT found CBD (75–300mg/day) did not improve motor symptoms vs. placebo. Non-motor symptoms: stronger evidence. A 2020 study found CBD improved REM sleep behavior disorder (a common PD symptom) in 4 of 4 patients. Psychosis: CBD (150mg/day) significantly reduced psychotic symptoms in a 2009 RCT of PD patients with psychosis. Pain: observational evidence supports cannabis for PD-associated pain. Dyskinesia (levodopa-induced): CBD reduced dyskinesia in animal models; human evidence is limited. The evidence profile suggests cannabis may be more useful for non-motor PD symptoms (sleep, pain, psychosis) than for core motor symptoms.