Paroxetine

Antidepressants - SSRI

Drug Overview

Paroxetine is a selective serotonin reuptake inhibitor (SSRI) sold under brand names such as Paxil and Seroxat. It is commonly prescribed to help balance neurotransmitter levels in the brain.

Paroxetine is used to treat major depressive disorder, panic disorder, obsessive-compulsive disorder, social anxiety disorder, and post-traumatic stress disorder. By blocking the serotonin transporter, it increases the availability of serotonin in neural synapses, which helps improve mood and reduce anxiety symptoms.

Paroxetine works by blocking the serotonin transporter (SERT), which leaves more serotonin available between nerve cells. That extra signaling is the basis for the antidepressant effect.

Relevant Genes and Their Roles

Paroxetine is primarily metabolized by the liver enzyme CYP2D6. The CYP2D6 gene encodes a cytochrome P450 enzyme responsible for phase I metabolism, converting paroxetine into less active compounds.

Variations in the CYP2D6 gene can lead to differences in enzyme activity, ranging from no function to increased function. These changes affect how quickly paroxetine is cleared from the body, influencing both its efficacy and risk of side effects.

Impact of Genetics on Drug Response

Genetic variations in CYP2D6 categorize patients into ultrarapid, normal, intermediate, and poor metabolizers. Ultrarapid metabolizers clear paroxetine more quickly, reducing drug levels and potential benefit, while poor metabolizers clear it slowly, raising levels and risk of adverse effects. Intermediate metabolizers fall between these extremes, and normal metabolizers have expected responses. Indeterminate or untested genotypes follow standard dosing.

Expected Clinical Effects of Genetic Variation

Ultrarapid Metabolizer

  • Effect on drug levels: Significantly decreased plasma concentrations.
  • Clinical consequence: Reduced therapeutic benefit and risk of suboptimal response.
  • Side effects: Generally lower risk of side effects; mild gastrointestinal upset possible.

Normal Metabolizer

  • Effect on drug levels: Expected plasma concentrations.
  • Clinical consequence: Standard efficacy with typical onset of action.
  • Side effects: Common SSRI side effects (nausea, drowsiness), usually mild to moderate.

Intermediate Metabolizer

  • Effect on drug levels: Moderately increased plasma concentrations.
  • Clinical consequence: Potential for enhanced therapeutic effect but higher risk of side effects.
  • Side effects: Increased frequency and severity of SSRI-related effects (insomnia, sexual dysfunction).

Poor Metabolizer

  • Effect on drug levels: Markedly increased plasma concentrations.
  • Clinical consequence: High risk of adverse effects at standard doses.
  • Side effects: Severe (serotonin syndrome, sedation), may require dose reduction.

Indeterminate/Not Available

  • Effect on drug levels: Unknown.
  • Clinical consequence: No specific guidance; follow standard dosing with clinical monitoring.
  • Side effects: Unknown; monitor for typical SSRI adverse reactions.

Dosing Guidelines

The following dosing guidelines for paroxetine are based on CPIC recommendations for CYP2D6 metabolizer status.

CYP2D6 Dosing Guideline

Phenotype Clinical Consequence Guideline Recommendation
Ultrarapid Metabolizer Your body clears this drug very quickly, so another medication may work better. Select alternative drug not predominantly metabolized by CYP2D6.
Normal Metabolizer Your genetics suggest you process this drug normally, so standard doses are recommended. Initiate therapy with recommended starting dose.
Intermediate Metabolizer Your body processes this drug more slowly, so lower doses and careful adjustments are recommended. Consider a lower starting dose and slower titration schedule as compared with normal metabolizers.
Poor Metabolizer Your body processes this drug very slowly, so you may need half the usual dose to avoid side effects. Consider a 50% reduction in recommended starting dose, slower titration schedule, and a 50% lower maintenance dose as compared with normal metabolizers.
Indeterminate Unknown impact. Initiate therapy with recommended starting dose.
Not available Unknown impact. Initiate therapy with recommended starting dose.

Alternative Treatment Options

The CPIC guideline suggests selecting an alternative medication not primarily metabolized by CYP2D6 for ultrarapid metabolizers. Examples of such SSRIs include citalopram, escitalopram, and sertraline. These are provided as examples and not medical advice.

Sources and References

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Genes that affect Paroxetine

See the full list of drugs affected by each gene:

CYP2D6
Cytochrome P450 2D6

Brand names containing Paroxetine

See how your genetics affect each product Paroxetine shows up in:

Paxil
Brand of paroxetine

Related Guides

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

Disclaimer: This document is for informational purposes only and is not a substitute for medical advice. Clinical decisions should be made by a qualified healthcare professional.

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