The Journey of Fluoxetine

Fluoxetine is well absorbed from the gastrointestinal tract
following oral administration, with bioavailability of approximately 70-80% after first-pass
hepatic metabolism. Absorption is complete but food does not significantly affect it. Peak plasma
concentrations occur 6-8 hours after dosing, reflecting the drug's slow absorption from the GI
tract and rapid tissue distribution. The drug's high lipophilicity (logP 4.17) facilitates
membrane permeation throughout absorption. Unlike tricyclic antidepressants, fluoxetine is not
subject to significant anticholinergic-mediated motility changes that could alter absorption.
Capsule and liquid formulations are bioequivalent.

Fluoxetine has an exceptionally large volume of distribution
(12-43 L/kg), reflecting massive tissue accumulation — particularly in brain, lung, liver, and
adipose tissue. Brain concentrations are 20-50 times higher than plasma. The drug is highly protein-
bound (approximately 94%) to plasma albumin and other proteins. This large tissue reservoir is
responsible for its very long half-life and persistence after discontinuation. Fluoxetine
concentrates within serotonergic neurons, reaching the serotonin transporter (SERT) on pre-synaptic
terminals. The drug crosses the placenta and enters breast milk at concentrations approaching
maternal plasma levels. Its active metabolite norfluoxetine distributes similarly and accumulates
extensively.

Fluoxetine selectively inhibits the serotonin reuptake transporter
(SERT, SLC6A4) on presynaptic serotonergic nerve terminals. By blocking the secondary active
transport mechanism that recaptures 5-hydroxytryptamine (serotonin) from the synapse, fluoxetine
allows serotonin to persist longer in the synaptic cleft, repeatedly binding postsynaptic receptors.
The initial effect is simply increased 5-HT synaptic availability. However, the somatodendritic
5-HT1A autoreceptors initially dampen the response by inhibiting serotonergic neuron firing
(counteracting the benefit). With continued treatment over 2-4 weeks, these autoreceptors
desensitize, allowing full serotonergic transmission enhancement. Simultaneously, downstream
changes in gene expression occur: CREB phosphorylation, BDNF upregulation, and ultimately
hippocampal neurogenesis — believed to be the substrate of long-term antidepressant efficacy
rather than the acute serotonin increase itself. Fluoxetine also blocks 5-HT2C receptors,
contributing to its activating/anxiogenic side effects and weight neutrality.

Fluoxetine undergoes N-demethylation in the liver primarily by
CYP2D6 (major) and CYP2C9 to produce norfluoxetine, its primary active metabolite. Norfluoxetine
is an equally potent SERT inhibitor with a half-life of 4-16 days — far longer than the parent
compound (1-4 days). This active metabolite profoundly alters pharmacokinetics: the effective
half-life of the fluoxetine/norfluoxetine combination is 4-6 days in most patients, extending to
weeks in CYP2D6 poor metabolizers. Furthermore, fluoxetine is a potent mechanism-based inhibitor
of CYP2D6 and a moderate inhibitor of CYP2C9, CYP2C19, and CYP3A4. This CYP2D6 inhibition creates
important drug-drug interactions with codeine (blocks activation), tricyclics (increased toxicity),
antipsychotics, and tamoxifen (blocks formation of active endoxifen metabolite).

Fluoxetine and norfluoxetine are primarily excreted by the kidneys
as glucuronide conjugates. Approximately 60-80% of a dose appears in urine, with 15% in feces.
Due to the extended half-life (days-weeks), a washout period of 5 weeks is recommended before
switching to MAO inhibitors to prevent serotonin syndrome. This same property provides a buffer
against missed doses — a therapeutic advantage over shorter-acting SSRIs. In renal impairment,
fluoxetine half-life is not greatly prolonged, but caution is warranted; weekly dosing formulations
exist for maintenance.

Fluoxetine was the first SSRI approved (1987) and remains a
first-line treatment for major depressive disorder, OCD, panic disorder, bulimia nervosa, and
PMDD. Therapeutic effects typically begin at 2-4 weeks and continue improving through 6-12 weeks.
CYP2D6 poor metabolizers (7-10% of Caucasians) achieve plasma levels 2-3-fold higher and may need
lower doses. The QTc prolongation risk is low compared to tricyclics. Abrupt discontinuation is
well tolerated compared to shorter-acting SSRIs due to the self-tapering effect of norfluoxetine.