Inonotus obliquus is a parasitic fungus in the family Hymenochaetaceae, commonly known as chaga. It primarily grows on birch trees (Betula spp.) in cold temperate regions of the Northern Hemisphere, including Siberia, northern Europe, Korea, Japan, and parts of North America. The genus name Inonotus derives from Greek roots meaning “fibrous ear,” referring to the texture and shape of fruiting bodies in related species, while the species epithet obliquus is Latin for “slanted” or “irregular,” describing the asymmetrical, charcoal-like growth form of the fungus on host trees. Unlike typical mushrooms, chaga appears as a dark, cracked mass on the exterior of infected trees, while its true fruiting body develops internally or under bark.

Inonotus obliquus functions as a parasitic and eventually saprotrophic organism, infecting host trees through wounds and gradually decomposing lignin and cellulose. The visible chaga conk is a sterile structure composed of dense mycelium and host-derived compounds, particularly concentrated on birch trees, from which it derives characteristic chemical constituents. The fungus has been used historically in northern Eurasian folk traditions, especially in Russia and the Baltic region, where it is prepared as a decoction or tea-like beverage.

Inonotus obliquus is not consumed as a typical edible mushroom but rather as an extracted ingredient. The dried conk is broken into pieces and simmered to produce chaga tea, a dark, slightly bitter beverage with earthy and vanilla-like notes. It has also been incorporated into powders, extracts, and functional food products in modern contexts. Its use is primarily associated with traditional medicinal practices rather than mainstream culinary applications, although some cultures like the Ainu pair the bitter chaga tea with sweets.

Chemically, Inonotus obliquus contains a range of bioactive compounds, many of which are derived or modified from its birch host. These include betulin and betulinic acid (triterpenes originally from birch bark), polysaccharides such as β-glucans, melanins responsible for the dark coloration, and various phenolic compounds. These substances have been studied for antioxidant, anti-inflammatory, immunomodulatory, and potential anticancer effects in experimental models. The polysaccharides, particularly β-glucans, are associated with immune system interactions, while triterpenes contribute to broader pharmacological activity.