most anti depressants act by increasing the amount of seratonin or dopamine OR both. If your have naturally high levels of these neurotransmitters (or your depression/illness isn't really caused from low levels of the neurotransmitters, I would suspect that you could have a reaction caused by toxic high levels. Problems often arise because people are started on too high of a dose instead of working up to a healthy maintenence dose.
http://en.wikipedia.org/wiki/Serotonin_syndrome (synopsis)
Serotonin syndrome is a potentially life-threatening adverse drug reaction that may occur following therapeutic drug use, inadvertent interactions between drugs, overdose of particular drugs, or the recreational use of certain drugs. The terms serotonin toxicity or serotonin toxidrome are more accurate as they reflect the fact that it is a form of poisoning.[1][2] Rarely it may also be called serotonin storm, hyperserotonemia, or serotonergic syndrome.
Serotonin syndrome is a consequence of excess serotonergic activity at central nervous system (CNS) and peripheral serotonin receptors. This excess serotonin activity produces a spectrum of specific symptoms including cognitive effects, autonomic effects, and somatic effects. The symptoms may range from barely perceptible to fatal.[3] Numerous drugs and drug combinations have been reported to produce serotonin syndrome.
Diagnosis of serotonin syndrome includes observing the symptoms produced and a thorough investigation of the patient's history. The syndrome has a characteristic picture but can be mistaken for other illneses in some patients, particularly those with neuroleptic malignant syndrome. Treatment consists of discontinuing medications which may contribute and in moderate to severe cases administering a serotonin antagonist. An important adjunct treatment includes controlling agitation with benzodiazepine sedation. The high profile case of Libby Zion, who died from serotonin syndrome, resulted in changes to graduate medical education in the United States.
[edit] Signs and symptoms
Symptom onset is usually rapid, often occurring within minutes after self-poisoning or a change in medication. Serotonin syndrome encompasses a wide range of clinical findings. Mild symptoms may only consist of tachycardia, shivering, diaphoresis (sweating), mydriasis (dilated pupils), myoclonus (intermittent tremor or twitching), as well as overresponsive reflexes.[3] Moderate intoxication includes additional abnormalities such as hyperactive bowel sounds, hypertension and hyperthermia; a temperature as high as 40 °C (104 °F) is common in moderate intoxication. The overactive reflexes and clonus in moderate cases may be greater in the lower limbs than in the upper limbs. Mental status changes include hypervigilance and agitation.[3] Severe symptoms include severe hypertension and tachycardia that may lead to shock. Severe cases often have agitated delirium as well as muscular rigidity and high muscular tension. Temperature may rise to above 41.1 °C (106.0 °F) in life-threatening cases. Other abnormalities include metabolic acidosis, rhabdomyolysis, seizures, renal failure, and disseminated intravascular coagulation, these effects usually arise as a consequence of hyperthermia.[3]
The symptoms are often described as a clinical triad of abnormalities:[3][4]
Cognitive effects: mental confusion, hypomania, hallucinations, agitation, headache, coma.
Autonomic effects: shivering, sweating, hyperthermia, hypertension, tachycardia, nausea, diarrhea.
Somatic effects: myoclonus (muscle twitching), hyperreflexia (manifested by clonus), tremor.