There is no predictable way of estimating the amount of psilocybin in each mushroom. The amount is determined by the strain, size and age of the mushroom, and how they have been stored. The biggest danger associated with magic mushrooms use is probably misidentification. Some mushrooms are poisonous and cause stomach pains, vomiting and diarrhoea. Don't use mushrooms alone.
Magic mushrooms can cause panic attacks.
Because they can impair judgement accidents can occur. Do not drive while under the influence. If you do feel sick shortly after taking magic mushrooms, go straight to hospital. Take a sample of the mushrooms with you to help with identification. Although it is against the law to possess psilocybin, the possession and use of magic mushrooms in their natural form is not illegal.