The CYP2D6 gene encodes a liver enzyme that plays a role in metabolizing multiple SSRIs. There are many different genetic variants that affect enzyme activity and drug metabolism.

Poor Metabolizers (4/5 genotypes): These individuals lack functional CYP2D6 activity which results in slower metabolism of SSRIs like paroxetine and fluoxetine. This also leads to elevated drug levels, which can cause side effects such as sedation, agitation, nausea, or emotional blunting. Poor metabolizers are more prone to intolerance or toxicity at standard doses.

Ultrarapid Metabolizers (multiple copies of 1 or 2 alleles): These individuals have increased CYP2D6 activity, metabolizing SSRIs too quickly. This leads to low plasma concentrations and reduced therapeutic effect. Individuals with this genotype may be mistaken as treatment resistant due to poor response at normal doses.