There are around 4000 species of fungi in the UK and of these around 200 are known to be edible and 250 are known to be toxic. The vast majority haven't had thorough toxicological analysis. As some fungi are deadly poisonous, any species not known to edible or of uncertain identity should be treated as poisonous.
There are no fool-proof rules that can guide in the identification of edible or poisonous mushrooms. Although there are various folk-lore suggestions for doing this, none are reliable and many are simply nonsensical. The only way to be able to tell if a particular fungus is edible is to be able to correctly identify its species. There are no short cuts.
This is the most dangerous form of mushroom poisoning and accounts for most mushroom related deaths. Toxins in this group are found in the Death Cap, Destroying Angel, Marginate Pixycap and some of the Lepiota and orange-coloured Webcap species.
The symptoms start with gastric upset from 6-24 hours after ingestion. After this there is a period of apparent well-being before liver and kidney failure. Death can occur within 6 days.
The severity of the symptoms and prognosis depend on how much of which species have been ingested and the time taken to receive medical assistance.
This toxin is found in the Turban fungus (also known as the False Morel). If eaten raw or under cooked the symptoms and progression are similar to that described for cyclopeptides above. The False Morel is eaten in some parts of Europe - after double boiling and discarding the water each time. I wouldn't recommend trying this as there is evidence that poisoning can occur even if it is well cooked. The effects may also be cumulative - so poisoning may occur after eating this mushroom even if the person had previously eaten it without symptoms.
This toxin causes increased sweating, weeping and salivation. It may also result in gastric upset, blurred vision and low blood pressure. Deaths from this type of poisoning have been recorded, but are rare. With appropriate medical treatment a full recover is usually made.
This toxin occurs in a range of fungi including the Fly Agaric, Cream Clot (and some other Funnels), Red-staining Fibrecap (and other Fibrecaps).
Blood cell-damaging toxins
This type of poisoning is mainly caused by the Blusher and the Grisette (and its relatives). These toxins are destroyed by heating and so poisoning only occurs when eating these species raw or undercooked.
The effect is to destroy red blood cells and so cause anaemia. In extreme cases, the high number of damaged cells can cause kidney blockage.
These are the main active chemicals found in the so-called 'magic mushrooms'. These toxins result in mental disturbance rather than any physical symptoms. However, if taken unknowingly, the onset of these symptoms can cause considerable distress.
Ibotenic acid, Muscimol, Muscazone
These compounds are found mainly in species - Fly Agaric and Panther. They are mainly psychoactive in nature although there may be some initial gastric upset as well.
This is a catch-all section for toxins, not falling into the other groups, which cause gastric problems. Fungi containing toxins in this group include the Yellow Stainer, Sickener and Livid Pink Gill.
Generally, these poisonings are not life threatening. However, medical assistance should still be sought if this type of poisoning is suspected
Although not strictly poisoning, mushroom allergies may give similar symptoms to a true mushroom poisoning. The distinction is that the mushroom in question does not contain any actual toxins and may be eaten safely by all except those who are sensitive to it. Some people are allergic to normal shop-bought mushrooms, while others may find that they have allergies to certain wild species.