It is always important to inform any doctor you see of all medications you are taking, including any herbal or over-the-counter supplements.  Although many medications can be taken concurrently, there is potential for reactions between many medications as well, thus consideration must be given for this.  Sometimes, the potential reaction is minimal and may be due to additive side effects (e.g., sedating affects may combine).  Other times, the presence of one medication can influence the elimination of the other medicine from the body, either allowing excessive accumulation or causing too-rapid depletion.  Consequences can thus be toxicity or lack of efficacy.  The SSRIs have specific enzyme groups that metabolize the medication.  Each SSRI has a different profile as to the enzymes involved in its own metabolism.  MAOIs are generally contraindicated in combination with all other antidepressants due to the risk for serotonin syndrome, which can be fatal (although there are certain combinations that skilled clinicians can prescribe in a methodical way to minimize the risks).  Serotonin syndrome occurs when there is scess serotonin in the central nervous system.  Symptoms include tremor, confusion, incoordination, sweating, shivering, and agitation.  Most SSRIs are contraindicated in combination with thioridazine (Mellaril) as well, due to risk of cardiac toxicity.  SSRIs should be used caustiously in combinations with sibutramine (Imitrex), commonly prescribed for migraine, also due to risk for serotonin syndrome.  St. John's wort, an herbal preparation used for depression, should be avoided when on a prescribed antidepressant, also due to potential risk for serotonin syndrome.  Again, there are some circumstances when a psychiatrist will combine two SSRIs, for example, but this is typically done cautiously and under his or her guidance.