[link|http://www.sfweekly.com/issues/2002-08-28/bayview.html/1/index.html|Serotonin poisoning]
Excerpt:
Dr. Michael Persinger, a psychologist at Laurentian University in Canada, found in 1993 that meditation induces epilepsylike brain seizures in some people. His study of 1,081 students showed that the 221 meditators among them had a higher rate of hallucinating floating spots of light, hearing voices, and even feeling the floor shake. Other studies reported that meditators complained of feeling emotionally dead and seeing the environment as unreal, two-dimensional, amorphous. Those results aren't surprising if meditation reduces blood flow to the parietal lobe. In longtime meditators, unreality can strike spontaneously. Singer describes it as "involuntary meditation." One of her patients took anti-seizure medication for 25 years after quitting meditative practice to regain control of his mind.
Other side effects fall under the paradoxical umbrella of "relaxation-induced anxiety," or RIA. Instead of relaxing during meditation, RIA sufferers feel distressed. Psychologists at Virginia Commonwealth University monitored 30 chronically anxious people during guided meditation. Seventeen percent indicated that their anxiety got worse. A previous study led by Dr. Frederick Heide at Pennsylvania State University reported that the same happened to 54 percent of the subjects. Symptoms of RIA include panic attacks, sweating, a pounding heart, spasms, odd tingling sensations, and bursts of uncontrollable laughter or tears. RIA can also aggravate conditions, such as schizophrenia, depression, asthma, and bleeding ulcers, that were previously stable.
What physiological changes explain RIA? During meditation, the brain releases serotonin. People with mild depression might enjoy the increased levels of serotonin because the neurotransmitter can ease their mood. Drugs like Prozac mimic this effect. However, too much serotonin can cause all of the symptoms of RIA, according to Dr. Solomon Snyder, head of Neuroscience at Johns Hopkins University. In some cases of schizophrenia, an excess of serotonin coupled with meditation can drop-kick someone into psychosis.