Mushroom supplements, including popular varieties like reishi, lion’s mane, chaga, and cordyceps, are generally well tolerated. But several groups of people face real risks from taking them. If you have an autoimmune condition, take blood pressure or diabetes medications, have kidney problems, or are pregnant or breastfeeding, mushroom supplements could cause serious complications.
People With Autoimmune Conditions
Medicinal mushrooms are marketed for “immune support,” and that’s precisely the problem for anyone with an autoimmune disease. These supplements contain compounds called beta-glucans that stimulate immune activity. If your immune system is already attacking your own tissues, ramping it up further can worsen symptoms.
Lion’s mane is specifically flagged for people with multiple sclerosis, lupus, rheumatoid arthritis, and pemphigus vulgaris (an autoimmune skin condition). But the concern extends to most medicinal mushroom species, since immune stimulation is one of their primary biological effects. Reishi, turkey tail, and maitake all activate similar immune pathways. If you take immunosuppressive medications to manage an autoimmune condition, mushroom supplements could work against those drugs.
People on Blood Pressure Medication
Reishi mushroom can lower blood pressure on its own. If you’re already taking antihypertensive drugs like ACE inhibitors, calcium channel blockers, or diuretics, adding reishi creates a risk of your blood pressure dropping too low. Symptoms of excessive blood pressure reduction include dizziness, fainting, blurred vision, and fatigue.
This also applies to anyone who naturally runs low on blood pressure without medication. If you tend to feel lightheaded when standing up quickly, reishi supplements could make that worse. Other mushroom species haven’t been studied as closely for blood pressure effects, but reishi is the one with the clearest documented risk.
People Taking Diabetes Medications
Several mushroom species can lower blood sugar. In a clinical trial, patients with type 2 diabetes who took 1,500 mg daily of an extract from the Agaricus blazei mushroom alongside their diabetes medications showed significantly improved insulin resistance after 12 weeks compared to placebo. Their levels of adiponectin, a hormone that helps regulate blood sugar, rose by 20%, while the placebo group saw a 12% decline.
That sounds like good news, but the danger is in the combination. If you’re already on medication that lowers blood sugar, stacking a mushroom supplement on top could push your levels dangerously low. Hypoglycemia can cause shakiness, confusion, rapid heartbeat, and in severe cases, loss of consciousness. Reishi, maitake, and cordyceps have all shown blood sugar-lowering effects in studies. If you use insulin or oral diabetes drugs, adding a mushroom supplement without adjusting your medication is risky.
People With Kidney Disease or Kidney Stones
Chaga mushroom poses a specific and serious threat to kidney health. Chaga is extremely high in oxalates, compounds that can crystallize in the kidneys and cause damage. In one documented case, a 69-year-old man who consumed 10 to 15 grams of chaga powder daily for three months developed acute kidney injury. A biopsy revealed calcium oxalate crystals deposited in his kidney tubules, along with tissue scarring and tubular damage.
High-dose chaga can cause what’s called oxalate nephropathy, where oxalate crystals essentially clog and damage the kidney’s filtering system. This can lead to both acute kidney injury and long-term chronic kidney changes, including scarring (interstitial fibrosis) and tissue wasting. If you have a history of kidney stones, chronic kidney disease, or reduced kidney function, chaga supplements are particularly dangerous. Even people with healthy kidneys should be cautious about high doses.
People With Mushroom Allergies
This one sounds obvious, but mushroom allergies aren’t always diagnosed before someone tries a supplement. The first documented case of anaphylaxis to lion’s mane involved a 43-year-old man who developed abdominal pain, diarrhea, hives, and swelling within minutes of eating the mushroom. Skin prick testing confirmed a true allergy. He was prescribed an epinephrine auto-injector and told to strictly avoid all products containing lion’s mane.
Allergic reactions to mushrooms can range from mild (itching, stomach upset) to life-threatening (anaphylaxis with airway swelling). If you have known allergies to mold or fungi, you’re at higher risk of reacting to mushroom supplements. Reishi spore powders are another concern, as inhaled fungal spores can trigger respiratory symptoms in sensitive individuals. Starting with a small dose and watching for any reaction is a reasonable approach if you’ve never consumed a particular mushroom species before, but people with known fungal allergies should avoid these supplements entirely.
Pregnant and Breastfeeding Women
The short answer is that nobody knows whether medicinal mushroom supplements are safe during pregnancy or breastfeeding, because the studies simply haven’t been done. There are no human trials evaluating reishi, lion’s mane, chaga, cordyceps, or other popular species in pregnant or nursing women. Without safety data, the risk is unknown rather than known to be low.
Given that these supplements actively modulate the immune system, lower blood sugar, and affect blood pressure, the potential for harm during pregnancy is plausible even if unproven. The immune system undergoes carefully regulated changes during pregnancy to protect the developing baby, and supplements that broadly stimulate immune activity could theoretically interfere with that process.
People Taking Blood Thinners
Reishi mushroom has antiplatelet properties, meaning it can reduce your blood’s ability to clot. If you take anticoagulant or antiplatelet medications, combining them with reishi could increase your risk of bruising or bleeding. This is especially important before any scheduled surgery. Most guidance suggests stopping reishi supplements at least two weeks before a surgical procedure to reduce the chance of excessive bleeding.
People Undergoing Cancer Treatment
The relationship between mushroom supplements and cancer is complicated. Lab studies on reishi have shown it can affect cancer cell growth in breast, prostate, lung, liver, and bladder cells through various mechanisms. Reishi compounds have been shown to interact with estrogen receptors in breast cancer cells and with androgen receptors in prostate cancer cells. One compound in reishi, ganoderol B, can bind to androgen receptors and suppress prostate cell growth driven by testosterone.
This might sound beneficial, but that’s exactly why it’s risky during active treatment. If mushroom compounds are interacting with the same hormone receptors and signaling pathways that your cancer drugs target, they could either amplify side effects or interfere with how your treatment works. Reishi is actually one of eight ingredients in an herbal mixture that has been used as an alternative therapy for prostate cancer, which underscores that these are biologically active compounds, not harmless extras. Anyone on chemotherapy, hormone therapy, or immunotherapy for cancer should treat mushroom supplements as a potential drug interaction, not a benign addition.
Quality and Contamination Risks
Beyond specific medical conditions, the supplement industry itself creates risk. Mushroom supplements are not regulated the same way as pharmaceuticals, and contamination is a real concern. Mushrooms are bioaccumulators, meaning they absorb heavy metals like arsenic, cadmium, lead, and mercury from the soil they grow in. Independent testing of mushroom powders routinely screens for these contaminants, and levels vary widely between brands.
If you do take mushroom supplements, choosing products that have been third-party tested for heavy metals and purity reduces this risk. Products labeled with certifications from organizations like NSF International or USP have undergone independent verification. Without that testing, you’re trusting the manufacturer’s quality control, which varies enormously across the supplement market.