How are CYP2C19 and fatigue related?

CYP2C19 is a cytochrome P450 enzyme that metabolizes many drugs, including certain antidepressants (e.g., citalopram, escitalopram), proton pump inhibitors (omeprazole), and the antiplatelet drug clopidogrel. Genetic variants (e.g., *2, *17) alter enzyme activity: poor metabolizers have higher drug levels, increasing the risk of side effects such as fatigue, drowsiness, or nausea. Ultrarapid metabolizers may experience reduced drug efficacy, for example with clopidogrel. A direct link between CYP2C19 and fatigue in the absence of medication is not established. The clinical value lies in pharmacogenetics: testing can guide dosing or drug choice. For fatigue workup, blood counts, vitamin levels, and medical history are far more relevant. The evidence is mechanistic – based on pharmacokinetics, not on studies with fatigue as a primary endpoint.