Seborrheic dermatitis (SD) is a skin condition that's more common in people with Parkinson's disease (PD) than in the general population: 
Prevalence
SD affects 18.6–59% of people with PD. 
Severity
SD in people with PD can be severe and difficult to treat with topical creams. 
Risk
SD may be associated with an increased risk of developing PD, and may even precede a PD diagnosis. 
Cause
SD may be caused by autonomic dysfunction, which is when the nerves that control automatic body functions aren't working properly. In PD, this can lead to over-secretion of oils from the skin's sebaceous glands. 

Treatment
Oral cannabidiol (CBD) therapy may be able to improve symptoms of both SD and PD. CBD has anti-inflammatory and neuromodulatory properties that may help with motor symptoms of PD.