What role does CYP2D6 play in fatigue?

CYP2D6 is a key liver enzyme involved in metabolizing about 25% of all prescription drugs and also some endogenous neurotransmitters (e.g., dopamine, serotonin). Its genetic variants classify individuals as poor, intermediate, extensive, or ultrarapid metabolizers. In the context of fatigue, the most established link is indirect: altered drug metabolism can lead to higher or lower drug levels, potentially causing fatigue as a side effect (e.g., with certain antidepressants, beta-blockers, or opioids). However, direct evidence that CYP2D6 genotype itself causes fatigue in the absence of medication is limited. Most studies are pharmacogenetic and not designed to assess fatigue as a primary outcome. Consumer DNA tests (e.g., NutriCare, Longevity ALL IN ONE) may include CYP2D6 variants, but their clinical utility for fatigue management is not well-supported. The evidence is 'mixed': plausible mechanistic links exist, but no strong RCTs or guidelines recommend CYP2D6 testing for fatigue alone. A comprehensive workup for fatigue should prioritize common causes like iron deficiency, thyroid dysfunction, and sleep disorders.