Prescribing Fluoxetine

Why Fluoxetine Still Matters

Fluoxetine is the drug that made the modern era of psychiatry possible. When Prozac launched in 1988, it was the first SSRI in the United States, and it changed the entire risk calculus of prescribing an antidepressant: here, at last, was an agent that could not be used to end a life the way a tricyclic could, that did not require blood levels or ECGs to dose, and that a primary care physician could prescribe without fear. Depression treatment left the specialist's office and entered general medicine. Every SSRI that followed, sertraline, paroxetine, citalopram, escitalopram, is, in a sense, a variation on fluoxetine.

Nearly four decades later it remains a genuine first-line antidepressant, not a legacy drug. It is the only SSRI with an FDA pediatric depression indication (down to age 8) and the best pediatric evidence base. It is the only SSRI FDA-approved for bulimia nervosa. It costs pennies: generic fluoxetine runs roughly 13 cents a pill, about $4 a month at a discount pharmacy, which quietly makes it one of the most adherence-friendly antidepressants in the formulary. And its signature pharmacokinetic quirk, an extraordinarily long half-life, gives it the gentlest discontinuation profile of any SSRI: patients who fear "getting hooked" on an antidepressant can be told, truthfully, that this is the one that tapers itself.

But fluoxetine is not a drug you can prescribe on autopilot. Its two defining features, the long half-life and the potent inhibition of CYP2D6 and CYP2C19, are simultaneously its greatest strengths and its most important hazards. The long half-life means the drug is still around six weeks after the patient stops it, so a switch to an MAOI or an interacting medication is not "clean" the day you write the discontinuation order. The enzyme inhibition means fluoxetine silently raises the levels of a long list of co-medications, and lowers the efficacy of prodrugs like tamoxifen and tramadol that need those enzymes to become active.

Part 1: Indications: Who Is Fluoxetine For?

FDA-Approved Uses

• Major depressive disorder (adults and children ≥8)
• Obsessive-compulsive disorder (adults and children)
• Panic disorder
• Bulimia nervosa: the only SSRI FDA-approved for this indication
• Premenstrual dysphoric disorder (PMDD): as Sarafem
• Bipolar depression (in combination with olanzapine, marketed as Symbyax)
• Treatment-resistant depression (with olanzapine)

The Evidence-Based Clinical Picture

Major depressive disorder: the bread and butter

Fluoxetine is a reasonable first-line SSRI for adult depression. Be honest with yourself about the effect size: pooled antidepressant data (Kirsch; Cipriani's 2018 network meta-analysis) put the average drug-placebo difference at the small end, an effect size around 0.3, with most of the total response attributable to placebo and natural course. Roughly half of patients are meaningful responders. Fluoxetine performs about as well as other SSRIs; escitalopram carries a slight, arguably illusory, efficacy edge in network meta-analyses. What separates SSRIs from one another in practice is not efficacy but tolerability, interactions, and withdrawal, and here fluoxetine's profile is favorable.

Pediatric and adolescent depression: this is fluoxetine's home turf

This is the single most important thing to know about the drug. Fluoxetine is the only SSRI with an FDA indication for pediatric depression (age 8+) and has the strongest supporting trial data. In the landmark TADS trial (439 adolescents; four arms: fluoxetine, CBT, combination, placebo), combination therapy was superior at 12 weeks, but by 36 weeks the fluoxetine and CBT monotherapy arms had largely caught up; at one year roughly 75% had responded and about two-thirds had remitted. Only four SSRIs (fluoxetine, escitalopram, citalopram, sertraline) have any positive pediatric evidence, and even that is mixed (Cipriani, Lancet 2016), but fluoxetine sits at the front of the line.

OCD

SSRIs work about as well as clomipramine here (earlier meta-analyses favored clomipramine; later data equalized them). OCD characteristically needs higher doses and longer trials than depression: push to 60-80 mg and wait 8-12 weeks before calling it a failure.

Bulimia nervosa: the eating-disorder specialist

Fluoxetine is the only FDA-approved SSRI for bulimia, and the optimal dose is high: 60-80 mg/day, well above the depression dose. Its relatively lower weight-gain liability is a genuine advantage in this population, where any weight-promoting effect can wreck adherence. The same 60-80 mg strategy applies to binge eating disorder (off-label but well-supported).

PMDD: the fast responder

Fluoxetine works for PMDD at low doses (10-20 mg) and, unlike in depression, works fast: relief within days, sometimes hours. This is why intermittent luteal-phase-only dosing is viable in PMDD. The proposed mechanism is distinct from ordinary reuptake blockade: fluoxetine appears to accelerate the conversion of progesterone to the neurosteroid allopregnanolone, which fits the rapid onset.

Panic disorder and PTSD

FDA-approved for panic; effective off-label for PTSD, where fluoxetine ranks among the top choices (with venlafaxine and paroxetine) and offers the best tolerability of the three. As in all anxiety disorders, start low: anxious patients are exquisitely sensitive to the early activation/jitteriness of SSRIs.

Somatic symptom disorder / illness anxiety (hypochondriasis)

Genuinely effective, about 44% response to fluoxetine monotherapy in the Fallon 2017 trial (started 10-30 mg, titrated toward 80 mg, mean effective dose ~40 mg), with CBT adding only a small increment.

Seasonal and non-seasonal depression with light therapy

In Lam's 2016 trial, fluoxetine 20 mg plus 10,000-lux morning light beat either alone (76% response combined vs. 50% light, 29% fluoxetine, 33% placebo), a useful reminder that light therapy is a legitimate combination partner.

Who Is Fluoxetine Especially Good For?

• Children and adolescents with depression (first choice)
• Patients with eating disorders (bulimia, binge eating disorder)
• Patients who fear antidepressant withdrawal or "dependence": the long half-life is your best reassurance tool
• Patients prone to missing doses: the long half-life makes fluoxetine forgiving of imperfect adherence
• Cost-sensitive patients: among the cheapest antidepressants available

Who Is Fluoxetine a Poor Choice For?

• Patients on CYP2D6-dependent medications: the single biggest reason to reach for a different SSRI (escitalopram, sertraline, or citalopram are cleaner). Think: someone on tamoxifen, an antiarrhythmic, a TCA, or a complex antipsychotic regimen.
• Patients who need a drug that can be stopped and cleared quickly: e.g., an imminent MAOI switch, or a woman who may need rapid changes around delivery.
• Highly anxious or activation-prone patients at standard starting doses: dose lower and slower rather than avoiding the drug.

Part 2: Before You Start: Workup and Candidacy

Fluoxetine requires far less baseline workup than lithium: no mandatory labs, no ECG for most patients, no serum levels ever. What it requires instead is a good history, aimed squarely at the interaction and safety questions.

The Pre-Start Checklist (Mostly History, Not Labs)

• Full medication reconciliation: the critical step. Screen specifically for CYP2D6/2C19 substrates (see Part 7): antipsychotics, TCAs, beta-blockers, tamoxifen, opioids like tramadol/codeine, antiarrhythmics. Fluoxetine's whole risk profile lives here.
• MAOI status: an absolute contraindication. Because of fluoxetine's long half-life, allow a 5-week washout after stopping fluoxetine before starting an MAOI (and 14 days after stopping an MAOI before starting fluoxetine).
• Bipolar screen: screen every depressed patient for a history of mania/hypomania before starting any antidepressant; SSRIs can precipitate a manic switch in undiagnosed bipolar disorder.
• Suicidality assessment: at baseline, and especially in patients under 25 (boxed warning; see Part 6).
• Bleeding risk: note concurrent anticoagulants, antiplatelets, or NSAIDs (SSRIs modestly increase GI bleeding risk).
• Seizure history: relevant particularly if treating bulimia, where seizure risk is already elevated.
• Pregnancy status/plans in women of childbearing potential.

Labs: No mandatory baseline labs for fluoxetine itself. Consider a metabolic panel and TSH as part of a general depression workup (to rule out medical mimics), not because the drug demands them. Consider a baseline and follow-up sodium in the elderly, who are at risk for SSRI-induced hyponatremia (SIADH).

Part 3: How to Start and Dose

Formulations

• Capsules/tablets: 10, 20, 40 mg (a 60 mg tablet also exists)
• Oral solution: 20 mg/5 mL, invaluable for pediatric dosing, for the anxious patient who wants to creep up slowly, and for fine taper work
• Delayed-release 90 mg weekly capsule: a once-weekly maintenance option made possible by the long half-life; convenient but the daily generic is far cheaper
• Combination with olanzapine (Symbyax) for bipolar depression / TRD

Starting Dose and Titration

• Standard start (depression, adults): 20 mg once daily, usually in the morning (fluoxetine can be activating and may disrupt sleep if taken at night; if a given patient finds it sedating, switch to evening).
• Anxious, panic-prone, elderly, or sensitive patients: start 10 mg (or even 5 mg via the liquid) for the first week to blunt early activation, then advance to 20 mg.
• Children/adolescents: typically start 10 mg daily, increase to 20 mg after 1-2 weeks; the oral solution helps.
• The general SSRI escalation logic: sit at 20 mg for about a month; if response is partial and tolerability is good, go to 40 mg for another month; reserve 60 mg for inadequate response.

Dose Ranges by Indication

Labeled maximum: 80 mg/day. In practice, some clinicians and case series have gone higher (a listserv survey reported doses up to ~220 mg in exceptional cases), exploiting fluoxetine's wide therapeutic index. This is off-label and not routine, but it explains why 80 mg is rarely a hard biological ceiling in resistant cases. Do not exceed the label without a clear rationale and documentation.

Onset of Effect

Expect the standard antidepressant lag: anxiety and sleep may improve first, but mood and anhedonia take ~4 weeks, sometimes 6-8. Counsel patients explicitly that early side effects precede the benefit, so they don't quit in week one. (PMDD is the exception; there the response is rapid.)

Part 4: Monitoring

Fluoxetine needs clinical monitoring, not laboratory monitoring. There are no serum levels to draw and no routine labs the drug mandates.

What to Monitor and When

• Suicidality and clinical worsening: most intensively in the first few weeks and after any dose change, especially in patients under 25 (boxed warning). Early, frequent contact in this window is the practical response to the warning.
• Activation / emerging mania or hypomania: watch for a manic switch, particularly in patients with any bipolar features.
• Response and side effects: reassess at ~2 weeks (tolerability, activation, early signs) and ~4-6 weeks (efficacy).
• Sodium in the elderly: consider a follow-up sodium a few weeks in, given SSRI-associated hyponatremia risk.
• Serotonin syndrome: be alert whenever another serotonergic agent is added (see Part 6).
• Growth/weight in children on long-term treatment (a general pediatric SSRI consideration).

No ECG is required for fluoxetine in the absence of specific cardiac concerns. Unlike citalopram/escitalopram, fluoxetine carries no meaningful QTc liability.

Part 5: Side Effects and How to Manage Them

The governing principle mirrors the SSRI class: most side effects are either early-and-transient or dose-related; the ones that persist (sexual dysfunction, emotional blunting) are manageable with dose reduction or a switch. Warn patients that the earliest effects fade: that single sentence prevents most week-one dropouts.

Early and Transient (First 3-7 Days)

Nausea, loose stools/diarrhea, headache, upset stomach, and jitteriness/activation. These typically settle within a week as the gut and CNS adapt.

Management: take with food; reassure and wait; start at 10 mg in sensitive patients. Because fluoxetine can be activating, dose in the morning and, if insomnia appears, address sleep directly rather than reflexively stopping the drug.

Sexual Dysfunction: the Most Common Persistent Complaint

Affects a large share of patients (reported ranges run high across the SSRI class, on the order of 50-70% when actively asked about): decreased libido, delayed or absent orgasm, erectile dysfunction, genital anesthesia. It usually does not remit with time, and patients rarely volunteer it, so ask directly.

Management:

• Lower the dose if efficacy allows.
• Switch to a less offending agent (bupropion or mirtazapine have low sexual burden; among serotonergics, response varies by individual).
• Add bupropion as an antidote/augmenter (also mood-additive).
• Sildenafil/tadalafil for erectile dysfunction.
• A scheduled drug holiday is sometimes used with shorter-acting SSRIs but is futile with fluoxetine: the long half-life means there is no meaningful washout over a weekend.

Emotional Blunting / Apathy

A real, underrecognized, dose-dependent effect. At therapeutic doses fluoxetine preferentially blunts negative emotions (often the therapeutic goal); at higher doses patients can feel that all emotion has gone gray or flat, the "zombie" feeling, or that they've lost their "rough edges."

Management: this is largely avoidable with appropriate dosing. Lower the dose; most patients on sensible doses do not experience personality change. If blunting persists at an effective dose, consider a switch (bupropion is often favored for this reason).

Weight

A relative strength of fluoxetine. It has less weight gain than most other SSRIs (Uguz 2015), and is the only SSRI shown in humans to reduce stress-induced (particularly carbohydrate) eating. Some weight gain still occurs with long-term use in a minority, but among SSRIs fluoxetine (with sertraline and vortioxetine) sits at the low-risk end. This is a decisive advantage in eating disorders.

Sleep and Activation

Fluoxetine is on the activating end of the SSRIs. Insomnia and restlessness are common early; conversely a minority find it sedating. Dose in the morning by default; move to evening only if the individual patient reports sedation.

GI and Bleeding

Beyond early nausea/diarrhea, SSRIs modestly raise GI bleeding risk through impaired platelet serotonin, clinically relevant mainly with concurrent NSAIDs, anticoagulants, or antiplatelets. Co-prescribe a PPI in higher-risk patients.

Hyponatremia (SIADH)

Uncommon but important in the elderly and those on diuretics; presents as confusion, lethargy, falls, or headache. Check sodium if these appear.

Bruxism, Sweating, Dry Mouth

Class effects. Bruxism can be helped by low-dose buspirone; excessive sweating sometimes responds to dose reduction or an alpha-agonist.

Fatigue / Cognitive Complaints

Less prominent with fluoxetine than with paroxetine. If a patient reports cognitive dulling, check for emotional blunting, hyponatremia, or a mimicking medical cause before blaming the drug.

Part 6: Overdose, Toxicity, and Boxed Warnings

Overdose

This is one of fluoxetine's quiet virtues and a big part of why SSRIs displaced tricyclics. Fluoxetine has a wide therapeutic index and is rarely lethal in overdose taken alone. Overdose typically produces tachycardia, tremor, agitation, nausea, and, at high ingestions, seizures; management is supportive. The contrast with a TCA overdose, cardiotoxic and frequently fatal, is stark, and a legitimate consideration when prescribing to a patient with suicide risk.

Serotonin Syndrome

The most important acute toxicity, occurring when fluoxetine is combined with other serotonergic agents (MAOIs, other antidepressants, tramadol, triptans, linezolid, methylene blue, St. John's wort, dextromethorphan). The triad: mental status change, autonomic instability (fever, tachycardia, diaphoresis), and neuromuscular hyperactivity (clonus, hyperreflexia, rigidity). It is a clinical emergency.

Boxed Warning: Suicidality in Youth

Two practical lessons: monitor closely early, especially under 25, and combine CBT with fluoxetine when available, since it appears protective against SSRI-associated suicidality in youth, an argument for combination treatment.

Other Warnings

• Seizures: elevated risk in bulimia; use caution in patients with seizure disorders.
• Manic switch: can unmask bipolar disorder.
• Bleeding: as above.
• Hyponatremia: as above, chiefly in the elderly.

Part 7: Drug Interactions: Fluoxetine's Defining Hazard

If you remember one thing about fluoxetine beyond the half-life, make it this: fluoxetine is a potent inhibitor of CYP2D6 and CYP2C19. This, not any efficacy shortfall, is the main reason to reach for a different SSRI.

Drugs Whose Levels Fluoxetine Raises (via CYP2D6/2C19 Inhibition)

By shutting down these enzymes, fluoxetine slows the metabolism, and raises the blood levels and toxicity risk, of their substrates:

• Antipsychotics: aripiprazole, brexpiprazole, risperidone (and others). Levels can climb enough to cause EPS or sedation; dose reductions of 2D6-dependent antipsychotics are often needed.
• Tricyclic antidepressants: levels can rise substantially; monitor and consider TCA level checks.
• Bupropion: relevant when using it as an augmenter.
• Beta-blockers metabolized by 2D6 (e.g., propranolol, metoprolol).
• Certain antiarrhythmics (flecainide, propafenone): narrow therapeutic index; be cautious.
• Atomoxetine and other 2D6/2C19 substrates.

Drugs Fluoxetine Renders Less Effective (Prodrugs Needing 2D6 to Activate)

This is the mirror-image trap and easy to miss, because you'd expect an inhibitor only to raise levels:

• Tamoxifen: requires CYP2D6 to convert to its active metabolite endoxifen. Fluoxetine can blunt tamoxifen's anticancer efficacy. Avoid fluoxetine in patients on tamoxifen; choose a non-inhibiting SSRI (escitalopram, citalopram) or venlafaxine.
• Tramadol and codeine/hydrocodone: 2D6 converts these to their active opioid forms; fluoxetine can reduce analgesia (and, for tramadol, adds serotonin-syndrome risk on top).

Serotonergic and Other Key Interactions

• MAOIs: absolute contraindication; 5-week washout after fluoxetine, 14 days after an MAOI.
• Other serotonergics (triptans, tramadol, linezolid, methylene blue, St. John's wort): serotonin-syndrome risk.
• Anticoagulants/antiplatelets/NSAIDs: additive bleeding risk.
• QTc: fluoxetine itself is low-risk, but be cautious if it raises the level of a QT-prolonging 2D6 substrate.

The Practical Rule

Before prescribing fluoxetine, run the med list against CYP2D6/2C19. If the patient is on tamoxifen, an antiarrhythmic, a TCA, a 2D6-dependent opioid they rely on, or a complex antipsychotic regimen, strongly prefer a clean SSRI (escitalopram, citalopram, or at ≤150 mg, sertraline). For an otherwise healthy patient on few medications, the interaction concern is minimal.

Part 8: Special Populations

Pregnancy

SSRIs as a class are among the better-studied psychotropics in pregnancy, and untreated depression carries real risks to mother and fetus. Within the class, however, sertraline is generally the preferred SSRI in pregnancy and lactation, with the most reassuring data. Fluoxetine is not contraindicated and is reasonable to continue when it is the drug that has worked for a given patient, but if you are choosing fresh for a pregnant patient, sertraline is the more common first pick.

Fluoxetine's long half-life is a specific liability around delivery: the drug and its active metabolite cross the placenta and clear slowly, so neonates have higher exposure and a somewhat greater chance of transient poor neonatal adaptation (jitteriness, feeding/respiratory difficulty). As with all SSRIs late in pregnancy, counsel about neonatal adaptation syndrome and the small, debated PPHN signal, and weigh these against the substantial risk of relapse if treatment is stopped.

Breastfeeding

Again, sertraline (and paroxetine) are usually preferred for lactation because of lower milk transfer. Fluoxetine's long half-life means it and norfluoxetine accumulate more in breast milk and infant serum than shorter-acting SSRIs, so it is not first choice for a newly breastfeeding mother, though a mother already stable and breastfeeding on it, with a thriving infant, need not automatically switch. Monitor the infant for irritability, poor feeding, and sedation.

Elderly

• Start low (10 mg) and titrate cautiously.
• Watch for hyponatremia (SIADH) and falls/bleeding.
• Fluoxetine's activation can be useful in apathetic depression but disruptive if it worsens agitation or sleep.
• Fluoxetine does not carry citalopram's age-based QTc dosing restriction, which can make it (or escitalopram/sertraline) attractive, though the enzyme-inhibition caveats still apply in polypharmacy, which is the norm in this group.

Renal and Hepatic Impairment

• Renal: no routine dose adjustment; fluoxetine is hepatically metabolized. Standard care is fine across most renal function.
• Hepatic: because clearance depends on the liver, reduce the dose or extend the interval in significant hepatic impairment (e.g., cirrhosis); the already-long half-life gets longer.

Children and Adolescents

Fluoxetine's strongest population. FDA-approved from age 8 for depression and in pediatric OCD; the best-evidenced SSRI in youth depression. Start 10 mg, target 20 mg (higher for OCD), pair with CBT, and monitor suicidality closely per the boxed warning. The long half-life is a bonus given adolescent adherence patterns.

CYP2D6 Poor Metabolizers

Roughly 7% of people of European descent are CYP2D6 poor metabolizers and will clear fluoxetine even more slowly, amplifying both drug persistence and interaction potential. Titrate more gently in patients known to be poor metabolizers.

Part 9: Discontinuation: Fluoxetine's Great Advantage

This is where fluoxetine's long half-life pays its biggest dividend.

Fluoxetine has the lowest discontinuation-syndrome risk of any SSRI. Its extended half-life (and the even longer half-life of its active metabolite norfluoxetine) means the drug self-tapers: even after an abrupt stop, blood levels fall gradually over weeks, so the "brain zaps," dizziness, nausea, chills, and flu-like malaise that plague paroxetine withdrawal are largely avoided. Across the SSRI class, roughly 60% of patients have no discontinuation symptoms, ~35% mild-to-moderate, ~5% severe, and fluoxetine sits at the gentle end of that distribution while paroxetine sits at the harsh end.

Practical Uses of This Property

• For the withdrawal-anxious patient: fluoxetine's forgiving taper is a genuine selling point. Fear of "dependence" is a common barrier to accepting antidepressant treatment at all; fluoxetine defuses it.
• The "bridge" strategy: a well-known maneuver for patients struggling to come off a short-acting SSRI or SNRI (paroxetine, venlafaxine) is to switch them to fluoxetine, then taper the fluoxetine, letting the long half-life smooth the landing.

How to Stop

• After short courses, fluoxetine can often be stopped without a formal taper because it tapers itself.
• After prolonged use, still taper: expert guidance for long-term antidepressant discontinuation favors slow, hyperbolic reductions (on the order of ~20% steps, sometimes stretched over many months) to minimize both discontinuation symptoms and relapse.
• A kinetics caveat: because of fluoxetine's nonlinear, slow pharmacokinetics, reducing to 20% of a dose does not immediately yield 20% of the blood level; the washout lags. Factor that lag into any taper schedule.

Distinguish discontinuation syndrome (a self-limited withdrawal, days to a few weeks) from relapse of the underlying disorder (a return of the full illness that persists and worsens). The long tail can blur the two; a symptom that emerges weeks after stopping and keeps building is more likely relapse.

Part 10: Fluoxetine vs the Alternatives

Among SSRIs, three, fluoxetine, sertraline, and escitalopram, consistently rise to the top for the combination of medical safety, tolerability, and manageable withdrawal; citalopram, fluvoxamine, and paroxetine carry more baggage (QTc, interactions, or harsh withdrawal, respectively).

• vs Escitalopram: Escitalopram has a marginal (probably clinically trivial) efficacy edge and, crucially, no meaningful CYP inhibition, which makes it cleaner in polypharmacy and a better bupropion-augmentation partner. Escitalopram carries a dose-dependent QTc caution that fluoxetine lacks. For a straightforward adult on multiple meds, escitalopram is often the easier choice; for a child, fluoxetine wins on indication and data.
• vs Sertraline: Sertraline is the go-to in pregnancy, lactation, and cardiac disease, and has only low CYP inhibition below ~150 mg. Excellent all-around SSRI; preferred over fluoxetine in those specific populations.
• vs Paroxetine: Fluoxetine wins decisively on withdrawal (paroxetine is the worst) and weight (paroxetine gains more). Both are CYP2D6 inhibitors. Little reason to choose paroxetine over fluoxetine for most patients.
• vs Citalopram: Citalopram is clean on CYP interactions but carries the QTc dose ceiling (max 40 mg; 20 mg over age 60 or with 2C19 inhibitors). A trade-off between fluoxetine's interactions and citalopram's cardiac caution.
• vs Bupropion / vortioxetine: For patients where sexual dysfunction, sedation, weight gain, or cognitive dulling are the priority to avoid, bupropion and vortioxetine are often favored, but neither has fluoxetine's pediatric or eating-disorder niche.
• vs SNRIs / mirtazapine: Cipriani's data suggest a small efficacy advantage for some SNRIs and mirtazapine over SSRIs, affecting perhaps 1 in 24 patients, usually not enough to override tolerability and cost considerations first-line.

Mechanism: The Short Version

Fluoxetine selectively blocks the serotonin transporter (SERT), raising synaptic serotonin, with minimal effect on norepinephrine or dopamine reuptake (distinguishing it from SNRIs and TCAs). But transporter blockade is immediate while clinical benefit takes weeks, so the antidepressant effect is thought to arise from downstream adaptations: desensitization of serotonergic autoreceptors that initially restrain firing, and upregulation of BDNF-driven neuroplasticity and neurogenesis over the same ~4-week timescale on which patients report mood improvement.

Two pharmacologic facts define its clinical behavior:

1. A very long half-life: fluoxetine on the order of 1-4 days with chronic dosing, and its active metabolite norfluoxetine 7-15 days, which is why steady state, discontinuation, and interactions all play out over weeks, and why once-weekly dosing is even possible.
2. Potent inhibition of CYP2D6 and CYP2C19, the source of its interaction profile and its 6-week interaction tail.

A distinct mechanism operates in PMDD: fluoxetine appears to speed the conversion of progesterone to the calming neurosteroid allopregnanolone (by dramatically lowering the enzyme's Km), which explains why PMDD relief comes in hours to days rather than the usual weeks.

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