Reishi mushroom is generally well tolerated, but it’s not risk-free. In a survey of over 1,300 people taking reishi, about 9% reported side effects. Most were mild, like dry mouth and constipation, but rare cases of serious liver injury have been documented. Whether reishi is safe for you depends on your health status, what medications you take, and how long you plan to use it.
Common Side Effects
The most frequently reported side effect is dry mouth, affecting roughly 5% of users. Constipation follows at about 4%, then insomnia and itchy skin at around 3% each. These numbers come from a cross-sectional survey of cancer patients using reishi alongside conventional treatments, so the rates in otherwise healthy people may differ, but the pattern of mild, manageable side effects is consistent across the available literature.
There is no established safe dosage. Recommendations drawn from traditional Chinese medicine, expert opinion, and clinical trials range from 1.5 to 9 grams of dried extract per day. Most clinical studies have used around 3 grams daily for periods of 12 to 16 weeks. In the U.S., reishi is sold as a dietary supplement, meaning it doesn’t go through the same approval process as prescription drugs. In China, Japan, and Korea, some reishi preparations are classified as drugs requiring a prescription.
Liver Injury: Rare but Serious
The National Institute of Diabetes and Digestive and Kidney Diseases rates reishi as a “possible rare cause of clinically apparent liver injury.” Given how widely reishi is used around the world, liver problems from it are extremely uncommon, but the handful of documented cases are sobering.
Symptoms typically appear one to two months after starting reishi, though the range spans from a few days to six months. Warning signs include fatigue, nausea, abdominal pain, loss of appetite, dark urine, and yellowing of the skin or eyes. In most reported cases, stopping reishi led to full recovery within one to three months. One fatal case involved a 47-year-old woman who switched from boiled reishi slices to a powdered form and developed liver failure within two months.
Several of the documented cases share a detail worth noting: the person had recently switched to a new reishi product or a more concentrated powdered formulation. The specific compound responsible for liver damage hasn’t been identified, and contamination or adulteration of the supplement can’t always be ruled out.
Blood Thinning and Surgical Risk
Reishi can increase bleeding risk. It prolongs several measures of blood clotting time, which means it can amplify the effects of blood-thinning medications like warfarin. If you take any anticoagulant or antiplatelet drug, this interaction is clinically meaningful.
For anyone with a scheduled surgery, the standard recommendation is to stop taking reishi products at least seven days beforehand. This guidance comes from case reports of unexpected intraoperative bleeding in patients who had been using reishi supplements before their procedures.
Blood Sugar Effects
Reishi contains compounds that lower blood glucose through multiple pathways. Its polysaccharides stimulate insulin release, and its triterpenoids block enzymes involved in glucose metabolism. In one clinical study, 71 adults with type 2 diabetes who took reishi polysaccharide extract three times daily for 12 weeks saw significant reductions in both fasting blood sugar and long-term blood sugar markers.
This is potentially beneficial if you’re looking for blood sugar support, but it creates a real risk if you’re already taking diabetes medication. The combined effect could push blood sugar too low. Animal studies have shown reishi extracts dropping glucose levels by 30 to 55% depending on the preparation and dose, which illustrates how potent these effects can be.
Autoimmune Conditions
This is one of the most important and least discussed safety concerns. Reishi is widely marketed for “immune support,” and it does stimulate the immune system, but that’s precisely the problem for people with autoimmune diseases. A systematic review published in Lupus Science & Medicine identified reishi as one of 15 herbs with the strongest evidence for activating inflammatory immune pathways. Specifically, reishi triggers the same signaling cascades that drive autoimmune flares, boosting production of inflammatory proteins like TNF-alpha, IL-6, and interferon-gamma.
Reishi has been implicated in triggering or worsening autoimmune conditions including lupus. If you have lupus, rheumatoid arthritis, multiple sclerosis, or any other autoimmune disease, the immune-boosting properties of reishi work against you rather than for you.
Pregnancy and Breastfeeding
No clinical studies have evaluated reishi safety during pregnancy or breastfeeding. This isn’t a technicality. It means no one has systematically checked whether reishi affects fetal development, breast milk composition, or infant health. Given reishi’s documented effects on immune function, blood clotting, and blood sugar, the absence of safety data is a reason for caution, not reassurance.
Who Should Avoid Reishi
- People on blood thinners: Reishi extends clotting time and can increase bleeding risk, especially with warfarin or similar medications.
- People with autoimmune diseases: Reishi activates the same inflammatory pathways that drive autoimmune flares.
- People on diabetes medication: The blood-sugar-lowering effects of reishi can stack with your medication and cause dangerously low glucose.
- Anyone approaching surgery: Stop reishi at least seven days before any scheduled procedure.
- People with liver disease: While liver injury from reishi is rare, existing liver problems reduce your margin of safety.
- Pregnant or breastfeeding women: No safety data exists for these populations.
For generally healthy adults not taking medications in these categories, reishi at typical supplement doses appears to carry a low risk of serious harm. But “dietary supplement” is not a synonym for “harmless,” and the lack of standardized dosing, variable product quality, and limited long-term safety data all mean you’re working with incomplete information.