yes there are possible negative side effects from taking psilocybin/psilocin containing mushrooms...
while the chemical is hardly toxic (in amounts anywhere near and beyond a typical dosage - ie. there is nothing to worry about as far as physical toxicity is concerneD), there is a risk of negative long-term psychological effects.
taken from the vaults of erowid -
"Many people experience nausea and/or vomiting during mushroom experiences, epecially with higher doses. Other possible negative effects include anxiety and unwanted or frightening thoughts and visions. Mushrooms, though perhaps to a lesser degree than LSD, can precipitate strong, temporary changes in an individual's experience of life and reality. It can be a powerful psychoactive experience, especially at higher doses, which is significantly affected by experiences, set and setting. Recent experiences, especially strong ones, can have a substantial effect on a trip. Physically or psychologically unsettling events in the days before a Mushroom trip can blossom into more serious distress and trauma while tripping. It is important to be prepared for the possibility of encountering difficult or frightening mental states. The Psychedelic Crisis FAQ addresses some of the issues involved in dealing with a difficult trip."
"Addiction Potential
Mushrooms are neither physically addicting nor likely to cause psychological dependance. Many people actually find that their desire to use mushrooms goes down for a period of time after use. As with all substances, some people will use them more frequently than they are comfortable with. There is a short period of tolerance after Mushroom use. Using Mushrooms two days in a row is likely to lead to a diminished experience the second day, though spaced 5-7 or more days apart, this effect is nearly non-existent."
"Taking psyilocybin containing mushrooms while on an MAOI can dramatically increase the effects of the experience. MAOIs are most commonly found in the prescription anti-depressants Nardil (phenelzine), Parnate (tranylcypromine), Marplan (isocarboxazid), Eldepryl (l-deprenyl), and Aurorex or Manerix (moclobemide). Ayahuasca also contains MAOIs (harmine and harmaline). Check with your doctor if you are not sure whether your prescription medication is an MAOI."
"Individuals currently in the midst of emotional or psychological upheaval in their everyday lives should be careful about choosing to use psychedelics such as mushrooms as they can trigger even more difficulty."
"Individuals with a family history of schizophrenia or early onset mental illness should be extremely careful because mushrooms have been known to trigger latent psychological and mental problems."
so - yes there are risks (as with any drug use).
i personally don't think it's healthy to do mushrooms weekly... hell i wouldn't do them with any regularity (as much as i love them). however there is a real risk of overusing this drug as it can be so readily available.
eat them with care and respect - for even if there is no real physical risk, and you think the psychological risks are worth it (or aren't going to be a problem for you), the reality is - 1) you never know and 2) bad trips can be VERY VERY frightening, probably well beyond what you've ever experienced before.. a living nightmare. not to say thats a bad thing, but it well can be.
cheers,
jimbu