Can you tell a poisonous mushroom by colour or whether it splits cleanly?
― Poison shows no shape, survives boiling, and strikes late
"Dull colours are fine," "if it splits cleanly it's edible," "boil it with eggplant and the poison disappears." Every autumn you hear these mushroom sayings, and none of them can be trusted. Poison is a tiny chemical that has nothing to do with looks. Worse, some poisons survive boiling and don't start working until about half a day after you eat them.
An autumn mountain path. A fallen tree is covered in mushrooms that look like shiitake. Your companion says, "They split cleanly down the middle and they're a dull colour, so they're fine."
You take them home, grill them, and eat. The smell and taste are fine. Nothing happens by evening, and you think, "See, it really was fine."
But one of the most common causes of mushroom poisoning in Japan is exactly this kind of "dull mushroom that looks like shiitake." So why can't you trust rules based on appearance?
There are just two reasons the sayings fail
Poison is an invisible molecule inside the mushroom. It has nothing to do with the mechanisms that decide colour or how it splits. So some dull, cleanly-splitting mushrooms are poisonous, and some flashy ones are perfectly edible.
Some life-threatening poisons survive boiling, grilling, even drying. And they don't cause symptoms right away. By the time you think "I ate it and I was fine," the poison is already in your body.
In short, you can't choose by looks, and you can't cook the poison away. Let's go through this step by step.
Poison is a molecule with nothing to do with looks
A mushroom's colour comes from pigments on the cap's surface. Whether it splits cleanly depends on how the thin threads that make up its body are arranged. Neither property has anything to do with whether the mushroom makes poison.
In fact, the three species said to cause the most poisoning cases in Japan — Lampteromyces japonicus (tsukiyotake), Entoloma rhodopolium (kusa-urabeni-take), and Tricholoma ustaloides (kakishimeji) — are all drab brown or grey mushrooms. Tsukiyotake closely resembles shiitake and oyster mushrooms; kusa-urabeni-take closely resembles edible Entoloma relatives. And they grow in the same season, in the same kinds of places, as the edible species.
The most dangerous of all is the group of pure-white Amanita species related to the destroying angel. Far from looking drab, they can even look clean and harmless. This group is said to contain a poison that destroys the liver in tiny amounts. You can check just how small an amount is dangerous in Figure 1.
Poison that survives boiling and strikes late
The idea that "cooking makes it safe" can't be trusted either. The poison in the destroying-angel group is a small molecule made of eight amino acids joined in a ring. Because it's a ring, its shape is hard to break apart, and it is said to survive boiling, grilling, and drying. Sayings like "boil it with eggplant" or "pickle it in salt" have no power to remove the poison.
What's even scarier is how it acts. Look at Figure 2. For about 6 to 24 hours after eating, nothing happens at all. Then comes violent nausea and diarrhoea, which settles down again after about a day. But during that calm period, the poison keeps destroying liver cells bit by bit. How the person feels and what's happening inside the body don't match.
A mushroom nibbled by insects or slugs can still be poisonous to humans. Different bodies react to poison differently. If a poison works by stopping the liver's "protein-building job," creatures whose version of that job works differently from ours may be less affected by it.
So what should you actually do?
- Don't pick or eat wild mushroomsThere's no way to identify them by appearance rules. Even experts say telling similar species apart is difficult.
- Don't eat mushrooms someone gives you either, on your own judgementThe same goes for wild mushrooms given to you by someone else, or sold by the roadside. If in doubt, throw them away rather than eat them.
- If you've already eaten some, see a doctor even if you feel fineDon't wait for symptoms — bring the leftover mushrooms or a photo and get checked.
If you have nausea, diarrhoea, stomach pain, or a clouded consciousness, call 119 immediately. Even without symptoms, if you suspect it might have been a poisonous mushroom, see a medical professional right away rather than waiting to see what happens. Bringing any leftover mushroom, the uncooked mushroom, or a photo helps identify the species. Infants and elderly people dehydrate easily from vomiting or diarrhoea, so give them fluids while getting them to care quickly. Don't try to force vomiting on your own judgement — follow a doctor's instructions. Since heat doesn't destroy the poison, it's also important not to try to make it "safe" through clever cooking.
Summary
Mushroom sayings fail because the poison is a molecule with nothing to do with appearance. What's more, the deadliest poisons resist heat and don't start working until half a day or more after eating. The safest approach is to assume only cultivated mushrooms bought in shops are safe to eat.
Mushroom poison shows in neither colour nor how it splits.
The moment you feel "fine" is exactly when the poison is at work.
You can read about the mechanism behind mushrooms suddenly sprouting after rain in "Why do mushrooms suddenly sprout after rain?", and another example of food poisoning that heat can't prevent in "Why does curry left overnight cause food poisoning?".
- Buy one each of shiitake, oyster mushroom, and shimeji from a supermarket, and confirm that every one of them splits cleanly down the middle.
- Then compare photos of tsukiyotake and kusa-urabeni-take in a field guide or on a local government warning page. See how closely they resemble the mushrooms from the shop.
- Finally, try applying the "identification points" written in the field guide using only photos, and see how well they actually work.
Every shop mushroom splits cleanly, but so do many poisonous ones. You can confirm with your own hands that "does it split cleanly" tells you nothing. Wash your hands after touching wild mushrooms outdoors, and never put them in your mouth.
Want to know more? ― Terms, formulas, and textbook linksWe mark clearly which level each part belongs to, from junior high science to university specialist courses
- JHSCovered in junior high school science
- HSCovered in high school "Chemistry" or "Biology"
- HS+Advanced high school content, or textbook sidebar material
- UnivNot covered in high school — university specialist content (toxicology, biochemistry, mycology)
- ResearchNot yet settled even at university level — something researchers are actively investigating
JHSTerms: this phenomenon has names
- Natural toxin: a poison that an animal, plant, or mushroom already carries in its body. This is distinguished from food poisoning caused by bacteria (such as Clostridium perfringens) multiplying.
- Amatoxin: the general name for the poison found in the destroying angel and death cap groups. Said to be a molecule made of eight amino acids joined in a ring.
- Incubation period: the time from when a poison or pathogen enters the body until symptoms appear. For amatoxin, said to be around 6 to 24 hours.
JHSHSCheck it with a formula: how much poison is in a single bite
The amount of poison is "poison per gram of mushroom" multiplied by "amount eaten." We compare this to a danger threshold determined by body weight. The figures below are all rough estimates based on values reported in sources as "said to be." The meaning and units of each symbol are summarised in the table below the formula.
| In symbols | D = c × m , D_danger = k × W |
| In words | Poison entering the body = poison per gram × amount eaten. Danger threshold = danger amount per kilogram of body weight × body weight |
| Where this comes from | Poison increases in direct proportion to the amount eaten (proportional scaling). A poison's effect is determined by the amount relative to body size (body weight) — a basic principle of toxicology |
| Symbol | Meaning and unit |
| D | Poison entering the body (mg) |
| c | Poison per gram of mushroom (mg/g) |
| m | Amount of mushroom eaten (g) |
| k | Danger amount per kilogram of body weight (mg/kg) |
| W | Body weight (kg) |
| Poison per gram of raw mushroom (c) | said to be approx. 0.2 mg |
| Danger amount per kilogram of body weight (k) | said to be approx. 0.1 mg |
| Body weight (W) | Adult 60 kg, child 20 kg |
| Adult danger threshold | 0.1 × 60 = 6 mg |
| Child danger threshold | 0.1 × 20 = 2 mg |
| Poison in half a cap (30 grams) | 0.2 × 30 = 6 mg |
| Amount that reaches the child's threshold | 2 ÷ 0.2 = 10 g |
③ In practical terms, that works out to about half a cap for an adult, or a single bite (10 grams, about the weight of 2-3 sugar cubes) for a child, to reach the threshold. The actual poison content is said to vary many times over between individual mushrooms and parts of the mushroom. There's no way to know in advance what amount would be "fine."
HSHS+Why it resists heat, and why it targets the liver
HSAmatoxin is a cyclic peptide, amino acids joined by peptide bonds into a ring. Ordinary proteins unfold under heat and change their properties (denaturation). But a small ring-shaped molecule has no "end" to unfold from, so its shape holds even when heated.
HS+This poison is said to bind to the enzyme that copies genetic information (RNA polymerase II) and stop it from working. Once copying stops, the cell can no longer make new proteins. Blood absorbed from the intestine flows first to the liver, so liver cells are the first — and most heavily — exposed to the poison.
UnivA toxicologist's view: dose and circulation in the body
Comparing poison strength by dose relative to body weight is the basic approach in toxicology, using measures such as the median lethal dose (LD50). Amatoxin is said to undergo enterohepatic circulation: it's released from the liver into bile, then reabsorbed in the intestine. This lets the poison stay in the body for a long time. Treatment may involve measures to break this cycle, or drugs that block the liver from taking up the poison. In severe cases, a liver transplant may be required.
📖 For the derivation of the formula and further reading: α-Amanitin (Wikipedia, Japanese) / Lampteromyces japonicus (Wikipedia, Japanese)
ResearchWhat's still not fully understood
- There's still no reliably effective antidote. Several drugs that block uptake into the liver are in use, but how well each one actually works is said to be hard to test comparatively, and remains debated.
- Why mushrooms make poison at all. It's often said to be a defence against being eaten, but since some poisons barely affect insects, its true purpose isn't clear.
- Why poison content varies even within the same species. Poison amounts are said to vary widely by location, year, and individual mushroom, and what determines this is still being investigated.
In other words, even this article describes things only as far as they're currently understood. Even so, the conclusion — "don't judge a wild mushroom by its looks and eat it" — is expected to hold regardless of future research.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | Science: "Connections among living things (fungi, decomposers)," "digestion and absorption" | Mushrooms are fungi; what's absorbed from the gut goes to the liver |
| HS | Chemistry: "amino acids and proteins"; Biology: "gene expression" | Cyclic peptide, heat denaturation |
| HS+ | Advanced biology: "regulation of transcription" | The poison that stops RNA polymerase II |
| Univ | Toxicology, pharmacology, mycology | Dose, enterohepatic circulation, classification of poisonous mushrooms |
| Research | Clinical toxicology, chemical ecology | Antidote efficacy, why mushrooms make poison |
| ― | Everyday connections | Autumn mushroom hunting, handling mushrooms given to you by others |
- Ministry of Health, Labour and Welfare, "Risk profile of natural toxins (higher fungi: Lampteromyces japonicus, Entoloma rhodopolium, Tricholoma ustaloides, destroying-angel group, etc.)" (厚生労働省「自然毒のリスクプロファイル」)
- Ministry of Health, Labour and Welfare, "Food poisoning statistics" (causative agent breakdown, plant natural toxins) (厚生労働省「食中毒統計資料」)
- Tokyo Metropolitan Government Bureau of Public Health, "Food Hygiene Window," explanation of poisonous mushroom food poisoning (東京都保健医療局「食品衛生の窓」)
- Wikipedia, "α-Amanitin" (ウィキペディア「α-アマニチン」)
- Nagasawa Hidefumi (supervising ed.), Japan's Poisonous Mushrooms, Gakken (長沢栄史 監修『日本の毒きのこ』学研)
※This article is a general-audience science explainer. The figures given are estimates meant to help you understand the mechanism. For identifying mushroom species or treatment, follow the guidance of a doctor, public health centre, or local government. In an emergency, call 119.