Fluoxetine

Fluoxetine is a selective serotonin reuptake inhibitor (SSRI) sold under brand names such as Prozac and Sarafem. It is one of the most widely prescribed antidepressants and is used to treat major depressive disorder, obsessive-compulsive disorder, panic disorder, bulimia nervosa, and premenstrual dysphoric disorder.

Like other SSRIs, fluoxetine works by blocking the serotonin transporter (SERT), which leaves more serotonin available between nerve cells. One feature sets it apart from most other SSRIs: fluoxetine and its active metabolite norfluoxetine have unusually long half-lives, so the drug and its effects persist for weeks after a dose change or after stopping.

No. Fluoxetine does not have genotype-based dosing guidance of its own. Fluoxetine is cleared mainly by CYP2D6 and CYP2C9, but the major clinical sources do not issue an actionable, genotype-driven dosing recommendation for it. The CPIC guideline for SSRI antidepressants provides genotype-based dosing for medications such as paroxetine, fluvoxamine, citalopram, escitalopram, and sertraline, but concluded there was insufficient evidence to recommend genotype-guided dosing for fluoxetine.

Part of the reason is that fluoxetine inhibits the very enzyme that clears it, and its long-acting metabolite complicates the relationship between genotype and drug levels. For that reason your CYP2D6 metabolizer status does not translate into a clear starting-dose adjustment for fluoxetine the way it does for some other antidepressants. A Gene2Rx report therefore does not report a metabolizer-based recommendation for fluoxetine itself.

Fluoxetine matters for pharmacogenetics because of what it does to other medications. Fluoxetine and norfluoxetine are strong inhibitors of CYP2D6, the enzyme responsible for metabolizing a large share of psychiatric medications, several opioids, and other common drugs. Fluoxetine is also a moderate inhibitor of CYP2C19.

A strong CYP2D6 inhibitor can knock the enzyme's effective activity down to near zero. This means someone who is genetically a CYP2D6 normal metabolizer can function as a poor metabolizer while taking fluoxetine. This effect, where a drug changes your effective metabolizer status, is called phenoconversion. Because norfluoxetine has a half-life of one to two weeks, the inhibition can persist for weeks after fluoxetine is stopped.

When fluoxetine suppresses CYP2D6, it changes how the body handles medications that depend on that enzyme. The direction of the effect depends on whether a drug is deactivated or activated by CYP2D6:

• Drugs that build up: medications cleared by CYP2D6, such as tricyclic antidepressants, atomoxetine, risperidone, aripiprazole, and some beta-blockers, can reach higher levels and carry a greater risk of side effects.
• Prodrugs that lose effect: medications that need CYP2D6 to become active, such as codeine and tramadol (for pain relief) and tamoxifen (converted to active endoxifen), can become less effective because the activating step is blocked.

None of this is a reason to start or stop fluoxetine on your own. It is the kind of interaction your prescriber weighs when combining fluoxetine with other medications, and it is exactly the situation where knowing your underlying CYP2D6 genotype adds context.

• CPIC - Guideline for SSRI and SNRI antidepressants
• FDA - Table of substrates, inhibitors, and inducers (CYP2D6 strong inhibitors)

Learn more about how genetics may affect your response to Fluoxetine and related medications:

• Can Two Safe Medications Be Dangerous Together? How Metabolism Interactions Work
• Grapefruit Juice and Medications: Why Drug Labels Warn About It
• Medications That Cause Phenoconversion: The Common Enzyme Inhibitors
• What Is Phenoconversion? When Your Medications Override Your Genes