By HealthDataConsortium.org Research Team
Short answer: in the human studies pulled together in a 2025 systematic review, the lion's mane (Hericium erinaceus) side effects that were reported were mild: stomach discomfort or diarrhea, headache, nausea, or a skin rash. The same review describes one case report of a serious lung reaction in a man who was regularly taking lion's mane, and the case report points toward a possible immune or allergic cause. Nobody can give you a reliable “percent of people affected,” because the review says side effects are commonly unreported and the research was not built to measure that. This guide shows what has been reported, where the evidence stops, and gives you a printable log and a stop-and-call list.
This is general education, not medical advice. If you are unsure whether a symptom is serious, contact a clinician or pharmacist.
What Side Effects Have Been Reported With Lion's Mane?
The most useful single source we found is a 2025 systematic review in Frontiers in Nutrition by Menon and colleagues. It pulled together 26 studies on lion's mane as a supplement. Only some were human trials: the review counts 5 randomized controlled trials and 3 pilot clinical trials, and the rest were lab studies, a cohort study, a case report, and a computer analysis. Across the human studies, daily amounts ranged from 0.8 to 10 grams, for anywhere from 7 days to 49 weeks. The most common pattern was 1 to 3.6 grams a day for 4 to 16 weeks.
On safety, the review states: “Although commonly unreported, potential side effects of HE include stomach discomfort, headache, and allergic reactions” (HE is its abbreviation for Hericium erinaceus). The specific events it describes are:
- Stomach discomfort or diarrhea: in the Mori trial, the review counts 7 mild events in the lion's mane group and 6 in the placebo group, with no treatment needed. Because it showed up on placebo too, it is hard to say how much was due to the mushroom.
- Headache: reported by one participant in one randomized trial, described as mild.
- Nausea, abdominal discomfort, and skin rash: reported in a 2020 pilot trial by Li and colleagues, described as mild.
- A menstrual-cycle change (epimenorrhea): one participant reported it. That participant was excluded from the sample, and the review notes it was not attributed to lion's mane.
Is There Any Serious Reaction on Record?
Yes, but it is a single case. The same review describes a case report of a 63-year-old Japanese man with mild, untreated diabetes who was regularly taking lion's mane supplements until he was admitted to the hospital with acute respiratory distress syndrome, a serious lung injury. The report noted elevated lung-injury markers (serum SP-A and SP-D) and strong lymphocyte (immune cell) proliferation, and a positive lymphocyte reaction test, which the case's authors took as support for an immune-type reaction to the supplement.
Keep this in proportion. One case cannot show how often this happens, and it cannot prove lion's mane was the cause. What it does support is a practical rule: breathing symptoms after starting a mushroom supplement are never something to wait out.
How Common Are These Side Effects?
We could not find a trustworthy rate, and we would rather say that than invent one. Here is why:
- The review itself notes that side effects are commonly unreported, so the absence of a listed problem is not proof there were none.
- The review lists small sample sizes in the cognitive studies and limited generalizability across studies among its limitations.
- Doses ranged more than tenfold across the human studies (0.8 to 10 grams a day), so results from one trial may not carry over to the product and amount you are considering.
- As a matter of basic study design, small trials are least able to detect rare reactions, which is the category the lung case belongs to.
A fair reading is “mild when reported, with real uncertainty at the edges.” That is not the same as “proven safe for everyone.”
What the Evidence Cannot Tell You
- Long-term safety. Most trials ran weeks to months. The longest duration in the review was 49 weeks.
- Safety alongside your medicines. Our guide to lion's mane and medication interactions notes that no clinical drug interaction studies in humans have been conducted, and explains the theoretical concerns and which medication classes to ask about.
- Quality of your specific product. The FDA states that it is not authorized to approve dietary supplements for safety and effectiveness before they are marketed, so what is in the bottle is not pre-checked by a regulator.
Printable Symptom Log
If you start lion's mane, a log turns a vague worry into something a clinician can use. The National Institutes of Health Office of Dietary Supplements advises keeping a written record of the supplements you take, including product names, doses, how often, and why, and sharing it with your health care providers. Copy the template below into a notes file or print this page, and make one entry per symptom.
Product name and form (capsule, powder, gummy, etc.): ____________________
Start date: ____________________
Daily dose on the label (and what you actually took): ____________________
Other supplements and medicines you take (list all): ____________________
Entry template (copy for each symptom):
- Date: ______ Time you took the dose: ______ Dose that day: ______
- Symptom: ____________________ Time it started (how long after the dose): ______
- Severity: mild (noticeable, no change to your day) / moderate (disrupts your day) / severe (cannot do normal activities)
- How long it lasted: ______ Did it ease after skipping a dose or stopping? yes / no / not tried
- Notes (food eaten, new products, anything else different): ____________________
Two habits make the log more useful. Change only one thing at a time, because starting several supplements together makes it impossible to tell which one a symptom came from. And write the entry the same day, while the timing is still clear.
Stop and Call: Red-Flag List
This list is built from general safety guidance, not from lion's mane trials, which are too small to define thresholds. When in doubt, stop the supplement and call.
Call 911 or your local emergency number right away
MedlinePlus describes anaphylaxis, a severe allergic reaction, as an emergency that needs medical attention right away. Its listed symptoms include difficulty breathing, coughing, wheezing or high-pitched breathing sounds, difficulty swallowing, swelling of the face, eyes, or tongue, hives, chest discomfort or tightness, dizziness or lightheadedness, and loss of consciousness. Do not take another dose, and do not wait to see whether it passes.
Stop the supplement and contact a clinician promptly
- Breathing: a new cough, wheeze, shortness of breath, or chest tightness that begins after you start the product, even if it seems mild. The one serious case described in the review involved the lungs.
- Allergy: a rash, hives, itching, or swelling that appears after a dose.
- Digestive: nausea, vomiting, diarrhea, or stomach pain that is severe, keeps getting worse, or does not settle after you stop.
- Anything unexpected: symptoms you cannot explain, or any symptom in someone who is pregnant, nursing, has a serious medical condition, or takes prescription medication.
Mild symptoms
For a mild, brief stomach upset or headache, many people pause the product and watch what happens, and note it in the log. If it returns when you restart, tell a clinician or pharmacist rather than pushing through it.
Who Should Talk to a Professional First
Ask a physician or pharmacist before starting if you take prescription medicines, have an autoimmune or immune condition, have a history of allergic reactions, have asthma or another lung condition, are pregnant or nursing, or have surgery coming up. Our guide on preparing a medication list for a pharmacist conversation walks through what to bring.
What to Do Next
- Start the log on day one, and bring it to your next appointment.
- Keep the product label or a photo of it, so you know the exact ingredients and dose.
- If you have a bad reaction, tell your health care provider. The NIH Office of Dietary Supplements notes you can also contact the FDA at 800-FDA-1088 or through its online form, and notify the manufacturer using the contact information on the label.
Sources
Source pages opened on October 3, 2026:
- Menon AM, Jalal AJ, Arshad Z, Nawaz FA, Kashyap R. Benefits, side effects, and uses of Hericium erinaceus as a supplement: a systematic review. Frontiers in Nutrition. 2025. DOI 10.3389/fnut.2025.1641246. Source for the study counts, doses, durations, reported side effects, the case report, and the stated limitations. Details about the Mori and Li studies are as described in this review.
- MedlinePlus. Anaphylaxis. Source for emergency symptoms and the call-emergency instruction.
- NIH Office of Dietary Supplements. Dietary Supplements: What You Need To Know. Source for keeping a written record, telling providers, and reporting side effects.
- U.S. Food and Drug Administration. Information for Consumers on Using Dietary Supplements. Source for the statement that FDA is not authorized to approve supplements before marketing.
This article is independent educational content. It does not recommend, review, or sell any product, and it is not a substitute for advice from your own clinician.
