The chief active ingredient in psilocybe mushrooms, psilocybin is a
tryptamine psychedelic with effects which on average last around six hours or so - about half as long as an
LSD trip, although the duration of both drugs varies widely. A mushroom trip has many features in common with an
LSD trip, but a lot of people find it less harsh and more 'organic,' with stronger awareness of bodily sensations and
the awesome beauty of nature. It is also rather like being
stoned, but without the
muzziness.
Mushrooms containing psilocybin are listed as poisonous in some sources chiefly because they consider psychedelic effects to be a kind of poisoning. The toxicity of psilocybin is really very low1, with a factor of about six hundred separating the sort of doses which will get you tripping from the sort of dose which hypothetically might kill someone. However magic mushrooms have been known to cause stomach cramps and occasionally nausea, and have in fact been blamed for a small number of deaths2. Such toxicity as exists in psilocybe mushrooms probably has more to do with baeocystin and norbaeocystin3 than it has to do with psilocybin per se.
The chief dangers of psilocybin mushrooms are psychological, not physiological. People have been known to go crazy largely from eating too many mushrooms, and many people who haven't gone crazy have had a terrible time. Psilocybin sometimes leaves people panicky and uncomfortable, especially if they are indoors and especially with people they don't trust. If at all possible, it is highly recommended that if you take magic mushrooms you take them outdoors, preferably somewhere with plenty of trees and grass. It is also extremely important to make sure that you are in a good way, both physically and mentally, before taking them. The intense awareness of bodily sensations can make something like low blood sugar levels or an itchy scalp profoundly unpleasant, and there is a real danger that someone who is not altogether comfortable with themselves or the people they are with will find their trains of thought taking hellish excursions. Take care! The sheer intensity of any psychedelic experience can be inspiring and elational, but there is always a risk that it will go the other way.
The other main danger involved in experiments with psilocybe mushrooms is the risk of eating the wrong thing. For Europeans this is not as much of a peril as it is sometimes made out to be, since there are no seriously poisonous mushrooms that look anything like Liberty Caps - the worst you are likely to get is a bit ill, although if you really went for it and ate about a hundred of the wrong ones you probably could do yourself some real damage. American shrooms (and some of the less-well-known European varieties) may more closely resemble really poisonous species. I hardly need to say that you should never eat any mushroom without being really sure what it is. Once again: Take care!
Psilocybin and psilocin (another tryptamine present in psilocybe mushrooms, into which psilocybin is broken down in the gut) quite closely resemble the neurotransmitter serotonin. It is thought that their psychedelic action is likely to be the result of serotonin inhibition. All psilocin is generally flushed out of the body within about 24 hours.
See also Psychedelic Guide: Mushrooms, and psilocin.
1 A table listing the toxicity of many drugs as determined by the misleading and unnecessary, but still sort of informative LD50 test can be found at http://www.erowid.org/psychoactives/health/psychoactives_ld50s.shtml
2 Psilocybe semilanceata, the Liberty Cap - the most common variety of magic mushrooms in the UK and most of Europe - seems to have killed one person: See http://leda.lycaeum.org/Documents/Fatal_Poisoning_After_a_Group_of_People_Voluntarily_Consumed_Hallucinogenic_Mushrooms.10488.shtml
3 Two of the other indoles found in the mushrooms, also thought to be significantly psychoactive. For an extensive list of these and more general information on psilocybin mushrooms see http://leda.lycaeum.org/Documents/Psilocybe_Mushroom_FAQ.9182.shtml