Although the number of documented serotonin reuptake inhibitor (SRI) discontinuation reactions is increasing, to date no systematic studies have been completed; therefore the mechanism of action for these reactions is not clearly understood. However, several hypotheses have been proposed. Researchers have postulated that discontinuation events result from a sudden decrease in the availability of synaptic serotonin in the face of down-regulated serotonin receptors. In addition, other neurotransmitters, such as dopamine, norepinephrine, or gamma-aminobutyric acid (GABA), may also be involved, although little research in this area has been published. Individual patient sensitivity, i.e., genetics or cognitive mindset, may also be a factor in SRI discontinuation phenomena. Finally, experts have hypothesized that since some symptoms associated with paroxetine withdrawal are similar to those of tricyclic antidepressant discontinuation, they may be caused by cholinergic rebound.