S-fluoxetine is an enantiomer of fluoxetine, a potent and selective inhibitor of the neuronal serotonin-uptake carrier and a clinically effective antidepressant. R-fluoxetine and S-fluoxetine enantiomers are nearly equipotent at blocking serotonin reuptake. In all biochemical and pharmacological studies, the eudismic ratio for the fluoxetine enantiomers is also near unity. A study examining the relative contributions of CYP enzymes to the metabolism of S-fluoxetine, and R-fluoxetine found dramatic differences. These data led to discovery programs between Lilly and Sepracor for the individual enantiomers. S-fluoxetine was studied in a phase 2 clinical trial for the prophylaxis of migraine, however, development was discontinued.