Norfluoxetine is a N-desmethyl metabolite of fluoxetine which acts as a selective serotonin inhibitor via blocking the reuptake of serotonin. It is believed that the substitution on the phenoxy ring determines the affinity of this inhibitor to monoamine transporters. It is a more potent inhibitor of serotonin uptake than fluoxetine with a half-life in vivo of 5-7 days. Studies show that the S enantiomer of Norfluoxetine is 20-22 times more potent at inhibiting serotonin reuptake than the R enantiomer.