SSRI's prevent the re uptake of serotonin and thus flood the brain with more serotonin than normal. The risks of using SSRI's include dependency, addiction, harsh withdrawal symptoms and a physiological/immunological rejection of the substance by the body. In clinical studies, significant numbers of people experienced dizziness, fatigue and increased mood swings when taken the drug after a period of three to six months. This seems to suggest that SSRI's may be habit-forming and that the body comes to depend upon them for everyday functioning. An extremely small percentage of clinical testers experienced immunological responses, leading to seizures, and/or stroke-like symptoms.