Chaga is surrounded by some of the boldest claims in the mushroom world, often backed by impressive-sounding laboratory figures. This article takes a careful look at what has actually been studied, what hasn’t, what is known about safety, and what can reasonably be concluded.
A short history
Chaga (Inonotus obliquus) has been brewed as a tea in Siberia, Russia, Finland and other parts of northern Europe for centuries, sometimes as a substitute for tea or coffee when those were scarce. See Chaga in Northern European and Siberian Traditions.
It became known internationally in the twentieth century, partly through Aleksandr Solzhenitsyn’s novel Cancer Ward, in which characters discuss the chaga tea drunk by Russian villagers. That literary fame, and the Soviet-era scientific interest that followed, did much to shape chaga’s modern reputation, long before any rigorous human research had been done.
What is chaga?
What we call chaga is not a mushroom in the usual sense. It is a sterile mass, known as a conk or sclerotium, that grows on living birch trees: black and deeply cracked on the outside, rust-orange within. The fungus only produces its short-lived fruiting body after the host tree dies. See Why Chaga Is Not Technically a Mushroom and Chaga and Birch Trees.
What researchers study
Chaga contains dark melanin pigments; triterpenes such as inotodiol, and betulin and betulinic acid, which are related to compounds in birch bark; polysaccharides; and polyphenols. See Chaga Compounds Explained and Triterpenes in Mushrooms Explained.
The three tiers of evidence
Laboratory studies
These make up most of the chaga literature. Many apply chaga extracts or purified compounds directly to cells, often at concentrations that could not be reached in the body. Chaga is also frequently promoted with test-tube “antioxidant scores” such as ORAC values. These measure a chemical reaction in a test tube, not what happens in the body. The US Department of Agriculture withdrew its ORAC database for foods in 2012 for exactly this reason.
Animal studies
Animal studies, mostly in rodents, have tested chaga extracts and fractions, often at amounts and in forms unlike human use.
Human studies
This is where the chaga evidence is weakest. We are not aware of any large, well-designed placebo-controlled trials of chaga in people, and clinical reviews have noted that trials are still needed to establish whether laboratory findings apply to people at all.
A materials problem
Many products sold as chaga are not made from the wild conk at all, but from chaga mycelium grown on grain. This material has different chemistry, lacks compounds the fungus forms while growing on birch, and can be high in starch from the grain. Studies of one can’t be assumed to apply to the other. See Mycelium on Grain Explained.
Safety: the oxalate question
Chaga is very high in oxalates, natural compounds that can form crystals in the kidneys when consumed in large amounts. Several published case reports describe serious kidney problems in people who took large amounts of chaga powder over long periods:
- Japan, 2014. A 72-year-old woman who had been taking four to five teaspoons of chaga powder a day for six months developed kidney failure linked to oxalate deposits.
- Korea, 2020. A 49-year-old man developed end-stage kidney disease after years of taking chaga powder. His doctors measured the oxalate in his remaining chaga and found it extremely high, around 14 g per 100 g, and estimated his oxalate intake at several times that of a usual diet.
- Korea, 2022. A 69-year-old man who had taken 10 to 15 g of chaga powder a day, along with vitamin C supplements, for three months developed acute kidney injury. He recovered with hospital treatment.
Case reports can’t tell us how common this is, and all of these cases involved large amounts of powder taken for long periods. But oxalates are water-soluble, so they can also be present in teas and extracts. We think anyone considering chaga should know about this.
If you have kidney problems or a history of kidney stones, are following a low-oxalate diet, or take high-dose vitamin C, speak to your GP before taking chaga. Laboratory and animal studies have also suggested chaga might interact with blood-thinning and blood-sugar medicines; whether this matters in people is unknown, but if you take these medicines, talk to your GP or pharmacist first. Anyone with a mushroom allergy should avoid chaga. If you are pregnant or breastfeeding, seek advice before taking any mushroom supplement. Tinctures also contain alcohol.
Sustainability
Chaga grows slowly, only on birch, and almost all of it is wild-harvested. Demand has raised concerns about over-harvesting in some regions. See Chaga Harvesting and Sustainability.
What we can reasonably conclude
Chaga has a long history as a northern European and Siberian tea and a distinctive chemistry that has attracted a great deal of laboratory interest. But there is very little research in people, and laboratory findings, including antioxidant scores, don’t tell us what chaga does in the body. The evidence doesn’t support specific claims about what chaga products do in people, and no health claims are authorised for chaga in the UK.
What remains uncertain
- Whether any laboratory findings apply to people.
- Which compounds and preparations might matter.
- How much oxalate different chaga preparations contain, and at what intake it becomes a concern.
- How chaga might interact with medicines in people.
What better evidence would look like
Well-designed, placebo-controlled human trials of a clearly identified and characterised material, ideally wild conk rather than grain-grown mycelium, with oxalate content measured and safety monitored.
Our approach
Our chaga is wild-harvested from birch in Finland and extracted in hot water and alcohol. We describe it by what it is and how it’s made, not by what it might do, and we think customers should know about the oxalate question. For the wider picture, see The Complete Guide to Chaga and How Chaga Is Extracted.
References
- Kikuchi Y, Seta K, Ogawa Y, et al. Chaga mushroom-induced oxalate nephropathy. Clinical Nephrology. 2014;81(6):440–444.
- Lee S, Lee HY, Park Y, et al. Development of end stage renal disease after long-term ingestion of chaga mushroom: case report and review of literature. Journal of Korean Medical Science. 2020;35(19):e122.
- Kwon O, Kim Y, Paek JH, et al. Chaga mushroom-induced oxalate nephropathy that clinically manifested as nephrotic syndrome: a case report. Medicine (Baltimore). 2022;101(10):e28997.
Study titles are reproduced as published and are not claims made by Spore & Bloom.
