Shiitake Mushroom Side Effects and Benefits (Including Shiitake Dermatitis)

Shiitake mushroom side effects

Most people eat cooked shiitake with no problems at all. The side effects below are real but uncommon, and the most important one is easy to avoid by cooking them properly.

Shiitake dermatitis

The best-known side effect: an itchy rash of long, thin red lines like whip marks, usually on the back, chest, arms and legs. It’s covered in detail below.

Digestive upset and raised eosinophils

Some people get bloating, stomach pain or diarrhoea, mostly with concentrated powders rather than a normal portion in a meal. In a small Mayo Clinic study, 10 healthy people took 4g of shiitake powder a day for 10 weeks. Half of them developed raised eosinophils in their blood, and two had stomach symptoms at the same time, which settled once they stopped. The same paper describes an earlier cholesterol study in which 17 of 49 people dropped out because of a rash or abdominal discomfort.

Shiitake allergy and occupational asthma in growers

True food allergy to shiitake is rare. Handling raw shiitake can occasionally cause an allergic contact rash on the hands. The bigger risk is for people who grow them: shiitake release large numbers of spores indoors, and studies of mushroom workers describe cough, breathlessness and a lung inflammation called hypersensitivity pneumonitis. At one shiitake farm, all four workers developed symptoms. Italian doctors reported the first European case of shiitake-induced occupational asthma in 2014, and the attacks stopped once the worker avoided shiitake.

iCan you react to one mushroom and not another? Yes. Shiitake dermatitis comes from a compound in shiitake, so it doesn’t mean button mushrooms will cause the same rash. If you’ve had hives or swelling after any mushroom, ask your GP about an allergy referral.

If your lips, tongue or throat swell, or you struggle to breathe after eating any food, call 999.

Other rare problems

• Sun sensitivity: case reports link shiitake to skin that reacts more to sunlight.
• Bowel blockage: cooked shiitake can turn slippery enough to swallow whole, and their tough fibre keeps its shape in the gut. Two people in Malaysia needed surgery, and a 69-year-old man in Japan passed four pieces of sautéed shiitake after hospital treatment. Slice them and chew well.

Supplements, medicines and health conditions

Extracts are far more concentrated than the mushroom you cook. There’s little good research on how shiitake extracts interact with medicines, so if you take blood thinners (such as warfarin or apixaban), immunosuppressants (for example after a transplant or for an autoimmune condition), or any regular prescription, ask your pharmacist before taking one. The same applies if you’re having cancer treatment: tell your oncology team about any mushroom supplement.

Pregnancy, breastfeeding and children

Thoroughly cooked shiitake are an ordinary food. The NHS list of foods to be careful with in pregnancy names one mushroom, enoki, which must be cooked thoroughly because of a listeria risk, and the same habit of cooking mushrooms properly makes sense for shiitake. Concentrated shiitake extracts are a different matter: we’d avoid them in pregnancy and breastfeeding unless your midwife or GP is happy. We don’t recommend our products during pregnancy or breastfeeding, and all our products are for adults (18+) only.

Shiitake dermatitis: symptoms, cause and how long it lasts

Shiitake dermatitis (also called shiitake flagellate dermatitis or flagellate erythema) is a striking rash of raised, itchy, red lines and small bumps that look like the skin has been scratched or whipped. It usually covers the trunk and can spread to the arms, legs and face. Doctors recognise it from its look and from asking what you ate.

What causes it?

It is thought to be caused by lentinan, a natural carbohydrate (a beta-glucan) in shiitake that is broken down by heat. That’s why it follows raw or undercooked shiitake and not properly cooked ones. It isn’t caused by mould or a bad batch, and it isn’t a classic allergy you can test for: skin biopsies and patch or prick tests are usually unhelpful. French poison centres found that the more shiitake people ate, the longer the rash lasted.

When does it start and how long does it last?

• Onset: anywhere from 1 hour to 7 days after eating, most often around 2 days.
• Duration: 1 to 40 days in the French series, with a typical 10 days. People who ate a small amount had it for about 4 days, a medium amount 7 days and a large amount 15 days.
• Outcome: everyone in the French series recovered fully. A review of 50 published cases found it settled in about 12.5 days without treatment.

How common is it?

DermNet estimates that about 2% of people who eat raw or lightly cooked shiitake get the rash. Overall it’s rare, but becoming more often reported in Europe as shiitake become more popular. French poison centres logged 59 cases between 2014 and 2019, and 110 between 2015 and 2022, which made it the second most common of the newer types of mushroom poisoning there. France’s food safety agency says this may be an underestimate, because people and doctors don’t always connect the rash to shiitake. In the UK, Brighton dermatologists wrote in the British Journal of General Practice to urge GPs not to miss it.

It doesn’t only follow meals. In a 2021 trial where adults ate snack bars made with shiitake, the researchers reported dermatitis in 10% of people sensitive to the mushroom, and a 2026 case report linked shiitake dermatitis to a mushroom coffee containing shiitake.

Is shiitake dermatitis dangerous?

Usually not. It looks alarming, but it isn’t contagious and it settles without treatment. Rarely it’s more severe: in 2025 Australian doctors reported a man with fever, swollen eyelids and abnormal blood tests after raw shiitake, who needed a long course of steroids. DermNet has clinical photos to compare with; eczema looks different.

Treatment: what helps

• Stop eating shiitake, including powders, drinks and supplements that contain it.
• Ease the itch: a pharmacist can suggest an antihistamine or a soothing cream. The rash clears by itself; in the French series, steroids and antihistamines didn’t clearly shorten it, while a review of case reports suggested treatment may shave a couple of days off.
• Keep the rash out of strong sun, since shiitake has been linked to sun sensitivity.
• See a GP or call NHS 111 if you have a fever, mouth sores, blisters, a rash that keeps spreading, or you’re unsure what the rash is. Fever, diarrhoea and mouth ulcers have been reported alongside the rash in some cases.

Once you’ve had it, it can come back if you eat undercooked shiitake again, and doctors have described more severe reactions after repeated exposure. If you do eat shiitake again, only eat them fully cooked, or avoid them altogether if you’d rather be cautious. If you’re not sure whether an itchy rash is food-related or something else, our guide to stress rashes explains other common causes.

Shiitake, kidneys and sulphites

Is shiitake safe for your kidneys?

For most people, yes. The US National Kidney Foundation uses dried shiitake in recipes it labels low potassium and low phosphorus. Per 100g, USDA figures give raw shiitake about 304mg of potassium, dried 1,530mg and soaked and cooked 117mg. If you’ve been told to limit potassium or phosphate, check portions with your renal dietitian.

Sulphites in dried shiitake

Dried mushrooms are sometimes preserved with sulphur dioxide. In October 2025 Ireland’s food safety authority recalled a batch of dried shiitake with sulphur dioxide missing from the label. If sulphites affect you, buy packs with a full allergen list.

How to eat shiitake mushrooms safely (fresh and dried)

France’s food safety agency, ANSES, advises that shiitake should only be eaten thoroughly cooked, whether fresh, dried or powdered, and warns that a quick stir-fry may not be enough. In practice:

• Don’t eat them raw, in salads, as a garnish or barely warmed.
• Cook until soft all the way through. Sauté sliced caps for several minutes until tender and browned, rather than tossing them in a wok for a minute at the end.
• Simmer them in soups and ramen, rather than adding them to the bowl at the last moment.
• Dried shiitake: soak in warm water until soft (usually 20 to 30 minutes), slice, then cook them as you would fresh. Strain the soaking liquid and use it in stock or sauces that you simmer.
• Stems: they’re edible but tough and woody. Remove them and simmer them in stock, then strain.
• Easy ideas: simmered in miso soup or ramen, roasted, in risotto, or first into the stir-fry pan.
• Shiitake powder: stir it into dishes that cook, not into smoothies or cold drinks.
• Eating out: if a dish arrives with pale, firm, barely cooked shiitake, it’s reasonable to ask for it to be cooked longer.

Fresh vs dried shiitake

Choose fresh caps that are firm and dry. Throw them out if they turn slimy, wet and dark, or smell sour. Shiitake are a good way to add savoury depth to vegetarian cooking and to your fibre intake; see our high-fibre foods list for more ideas.

Shiitake nutrition per 100g

Figures from the US Department of Agriculture’s FoodData Central. “Cooked” means dried shiitake that have been soaked and cooked. %NRV is the share of the daily reference value used on UK labels.

Per 100g, cooked shiitake are high in copper, pantothenic acid and selenium (a food needs at least 15% of the NRV per 100g to be called a source, and 30% to be called high). Fresh shiitake per 100g are a source of pantothenic acid but contain much less copper and selenium. Under the authorised GB health claims for these nutrients:

• Copper contributes to the normal function of the immune system.
• Copper contributes to normal energy-yielding metabolism.
• Pantothenic acid contributes to a reduction of tiredness and fatigue.
• Pantothenic acid contributes to normal mental performance.
• Selenium contributes to the normal function of the immune system.
• Selenium contributes to the normal thyroid function.

These claims belong to the nutrients, not to shiitake itself. A normal portion is smaller than 100g, so treat shiitake as a useful part of a varied diet rather than a supplement.

Do shiitake contain vitamin D?

Only a little, unless they’ve been exposed to UV light. Mushrooms make vitamin D2 when exposed to sunlight or a UV lamp, and UV-treated mushrooms can keep more than 10µg per 100g if eaten before their best-before date. Standard shiitake provide well under 1µg. Some UK shops sell UV-treated mushrooms labelled “high in vitamin D”; check the pack. They are a useful extra, not a replacement for the NHS advice to consider a 10µg supplement in autumn and winter. See vitamin D in winter.

Shiitake benefits: what the research shows

Lists of “10 proven shiitake benefits” mostly rest on lab and animal studies. Here’s what has actually been tested in people.

The immune study is the one most often quoted, but it measured lab markers, not illness, and had no placebo group. For proven ways to support your immune system, see how to boost your immune system, and for the wider picture on mushrooms and inflammation, see mushrooms for inflammation.

Eritadenine, ergothioneine and beta-glucans

Eritadenine is linked to cholesterol research, mostly in animals. Ergothioneine is an antioxidant found at higher levels in mushrooms than other foods. Beta-glucans, including lentinan, are behind most of the immune research. Lab findings don’t show what a portion does in people; the small human trials above do. See also stress and cholesterol and the anti-inflammatory diet.

Are there any authorised health claims for shiitake?

No. We checked the Great Britain Nutrition and Health Claims Register: claims submitted for shiitake (Lentinula edodes), such as “contributes to natural immunological defences”, are listed as non-authorised. Shiitake can’t legally be sold in the UK with a promise to support immunity, and the same goes for every other mushroom. Our functional mushrooms UK guide explains the rules and compares eight mushrooms side by side.

Lentinan: a medicine in Japan, not a supplement

Lentinan is a purified beta-glucan made from shiitake. In Japan it has been approved as a medicine used alongside chemotherapy for stomach cancer. The evidence is mixed:

• A 2009 analysis of 650 patients from five trials found that adding lentinan to chemotherapy for advanced stomach cancer was linked to longer survival.
• A larger, later Japanese trial of 309 patients found no survival benefit, and time to treatment failure was shorter in the lentinan group.

Either way, a hospital drug made from purified lentinan is not the same as eating shiitake or taking a shiitake supplement. If you’re living with cancer, talk to your oncology team before taking any mushroom product. Turkey tail has a similar story; see turkey tail and the cancer question.

Shiitake supplements, extracts and mushroom coffee

Shiitake turn up in capsules, powders, multi-mushroom blends and mushroom coffees. A few things to know before you buy:

• They aren’t exempt from shiitake dermatitis. The 2026 mushroom coffee case shows that drinks and powders can trigger it. Stop at the first sign of a rash.
• Check what’s in the tub. Look for the part used (fruiting body or mycelium), the amount per serving and tested beta-glucan content. Our guide to the best mushroom supplements in the UK explains how to read a label.
• Ask a pharmacist if you take regular medicines, and avoid extracts in pregnancy and breastfeeding.
• Food first. Most of what shiitake offer comes from simply cooking them into meals.

If it’s a mushroom drink you’re after, reishi is the traditional evening choice. Our Reishi Mushroom Adaptogenic Complex is a reishi hot chocolate, not a shiitake product: reishi is a different mushroom. It contains a little natural caffeine from the cacao, so it isn’t caffeine-free. Like all our products, it’s for adults only, and we don’t recommend it during pregnancy or breastfeeding. Read more in reishi cacao benefits, mushroom hot chocolate compared, our reishi guide, mushroom coffee and mushroom tea.

What are shiitake mushrooms?

Shiitake (Lentinula edodes) are brown-capped mushrooms native to East Asia and now one of the most widely eaten mushrooms in the world. They have a rich, savoury (umami) flavour and are sold fresh, dried and as supplements. Dried shiitake are especially flavourful in soups, stocks and stir-fries.

Shiitake sit in the same “functional mushroom” family as maitake, lion’s mane, chaga and cordyceps, but unlike most of those, they are first and foremost a kitchen ingredient. For a quick comparison of the most popular supplement mushrooms, see lion’s mane vs reishi vs cordyceps.

Related guides

• Reishi for skin and patch testing.
• Which Hifas da Terra products contain shiitake.
• How to improve your gut health with food first.