To date, the only study on the effects of Chaga in humans is a case series (a study that looks at a group of people receiving the same treatment) from 1973.
In this study, 50 people with both gastrointestinal disease and psoriasis either consumed Chaga orally (1 tablespoon, 3 times per day) or applied it topically (the dosage and duration were not reported). The primary outcome was the improvement in psoriasis lesions. 38 participants were considered completely cured, 8 participants showed improvement, and 4 participants saw no effect.[1]
However, the results should be interpreted with caution. Considering the age of the study, the lack of a control group, the lack of randomization, and the lack of a standardized measurement of psoriasis disease progression, more human research is needed to determine the utility and safety of Chaga.