Paroxetine, also known by the trade names Aropax, Paxil, Pexeva, Seroxat, Sereupin and Brisdelle, was first marketed in the U.S. in 1992. Effective for major depression and various anxiety disorders, it quickly gained a sizable share of the antidepressant prescription market. By the late 1990s, paroxetine frequently was being associated with serious drug interactions and medication side effects. Most significantly, in a major Canadian epidemiological study examining the relationship between antidepressants and diseases, paroxetine was associated with a 620 percent increase in the rate of breast cancer in women who had taken it over a four-year period. Though re-analyses of this investigation discounted the magnitude of these findings, other studies have associated paroxetine with numerous side effects and adverse events not reported in clinical trials. Among these are effects on male fertility, birth defects, gestational hypertension, prolonged QT interval in infants, hyperprolactinemia, cognitive impairment in the elderly, autism, sexual side effects, weight gain, and suicidality, aggression, and akathisia in children and adolescents. Paroxetine has the highest inhibitory constant for the P450 2D6 isoenzyme of all antidepressants (Ki = 0.065-4.65 micromoles). This high affinity explains its high inhibitory interaction profile with substrates for 2D6. Paroxetine's potent 2D6 inhibition also implies that significant inhibition of the metabolism of 2D6 carcinogen substrates occurs which implies an increased probability of oncogenesis. Through 2D6 inhibition, tamoxifen metabolism is inhibited, which has been found to increase the risk of dying from breast cancer over a five-year period in women on both medications. Paroxetine also is a potent inhibitor of 3A4 with multiple 3A4 substrate interactions. Paroxetine has the highest known affinity for the serotonin transporter (0.13 nanomoles) of any currently used antidepressant. These characteristics and their potential negative consequences along with other adverse effects are considered and weighed against paroxetine's efficacious antidepressant and anxiolytic effects.