The safety data for Chaga is limited; there are few human studies available.
The main concern is that it contains high levels of oxalate (a compound commonly found in plants that can cause kidney stones).[1] While anecdotal evidence suggests it is relatively safe, there are case reports of oxalate-induced kidney disease (a condition that occurs due to the buildup of calcium oxalate crystals within the kidney) after long-term consumption of Chaga.[2][3]
Because there are few human studies, Chaga should not be used during pregnancy or breastfeeding.