There are no known adverse side-effects of using mushrooms, other than leaving you feeling fatigued and probably surrounded by a load of mad paintings and some half-eaten chocolate bars.
However, magic mushrooms and other psychedelics are powerful mental amplifiers. If you are feeling depressed, anxious, sick at the world, at the quality of modern television, at rampant consumerism, at life in general, do not take psychedelics.
Mushroom effects and your reaction to them are strongly determined by set and setting. Set is your mindset when taking the drug; setting is where you are.
The magic mushroom effect makes you enormously sensitive to your environment so you should always be in a safe, comfortable environment, preferably with a friend you trust.
Know Your Shrooms
One of the dangers associated with taking magic mushrooms has do with the fungi themselves. If you decide to go out looking for mushrooms to take to get high, be sure you are picking the right ones. Unless you are very familiar with the "right" kind of mushrooms to achieve the effect are looking for, don't attempt to find your own. Some species of poisonous mushrooms look very similar to the psychedelic type, and you want to avoid them.
Moldy Magic Mushrooms
Whether you have grown your own mushrooms or have acquired them elsewhere, check them carefully before you ingest them. Any shrooms that have brown or black mold on them should be discarded. Mushrooms that have gone bad can make you quite ill due to the bacteria they carry.
Don't Use Mushrooms Alone
Tripping alone for the first time is not recommended. An experienced and trusted friend should either join you, or should abstain and act as a sober sitter or guide to just be around and to help you if you start suffering a bad trip.
Bad Trips on Psychedelic Mushrooms
When you decide to use psychedelic mushrooms, you have no idea what kind of experience you are going to have with them. Your trip could be a feast for the eyes of bright colors and wonderful sights. You also run the risk of having a bad trip, and there is no way of knowing in advance what kind of experience you are going to have.
The fact that you have used the mushrooms in the past and had a good experience doesn't mean that you are protected from having a bad trip at any time you decide to use them. During a bad trip, you could feel extremely anxious and even have feelings of paranoia.
To lower the risk of having a bad trip, start by taking a low dose. Give yourself some time (approximately 90 minutes) to see how the shrooms are affecting you before taking some more.
Mixing Shrooms and Other Drugs
If you are taking any other medications, it's a good idea to stay away from magic mushrooms. The combination of alcohol and shrooms is also one you want to avoid; alcohol is a depressant and you probably don't want to be tripping as well as experiencing the effects of being drunk (slurred speech, lack of coordination, etc.) if you need to ask the sober friend who is supposed to be with you for help.
Magic Mushrooms and Flashbacks
Flashbacks days or even months afterwards are often cited as a common side-effect of hallucinogenic drugs. There's little evidence that these are a given. Anecdotal studies have shown that only about 15% of users report any phenomenon close to a 'flashback.'
Even though not all shroom users experience flashbacks, it doesn't mean that this phenomenon can't be disturbing to the individual experiencing them. Reliving the experience of hallucinating in between trips, especially without warning, is not necessarily a pleasant occurrence for the mushroom user.