CYP2C19: the enzyme that decides whether some common drugs work for you

What CYP2C19 does

The gene makes an enzyme in the cytochrome P450 family. Its job is to chemically alter certain drugs so the body can activate or clear them. For most medications this runs in the background and the exact speed does not matter. For a few drugs it matters, because the drug is either switched on or removed by this one enzyme.

The metabolizer groups

People are grouped by how much working enzyme they make. Normal metabolizers make the usual amount. Intermediate and poor metabolizers make less, so drugs this enzyme activates stay weaker and drugs it clears build up. Rapid and ultrarapid metabolizers sit at the other end. The group comes from the combination of gene variants, called star alleles, a person carries. When the data does not pin that combination down, the honest answer is Indeterminate, and that is what we report.

Medications where it matters

clopidogrel (Plavix)

Clopidogrel is a blood thinner that only works after CYP2C19 converts it into its active form. Intermediate and poor metabolizers make less of the active drug and get less protection against clots. The drug's FDA label carries a boxed warning about poor metabolizers, and CPIC gives specific guidance below.

If your result is	Published CPIC guidance
Intermediate Metabolizer	Per CPIC (A): "Avoid standard dose (75 mg) clopidogrel if possible. Use prasugrel or ticagrelor at standard dose if no contraindication."
Likely Intermediate Metabolizer	Per CPIC (A): "Avoid standard dose clopidogrel (75 mg) if possible. Use prasugrel or ticagrelor at standard dose if no contraindication."
Likely Poor Metabolizer	Per CPIC (A): "Avoid clopidogrel if possible. Use prasugrel or ticagrelor at standard dose if no contraindication."
Normal Metabolizer	Per CPIC (A): "If considering clopidogrel, use at standard dose (75 mg/day)"
Poor Metabolizer	Per CPIC (A): "Avoid clopidogrel if possible. Use prasugrel or ticagrelor at standard dose if no contraindication."
Rapid Metabolizer	Per CPIC (A): "If considering clopidogrel, use at standard dose (75 mg/day)"
Ultrarapid Metabolizer	Per CPIC (A): "If considering clopidogrel, use at standard dose (75 mg/day)"

omeprazole

Proton pump inhibitors like omeprazole are cleared by CYP2C19. Slower metabolizers reach higher drug levels from the same dose, which often means it works well and that a lower dose can be enough for long-term use.

If your result is	Published CPIC guidance
Intermediate Metabolizer	Per CPIC (A): "Initiate standard starting daily dose. For chronic therapy (>12 weeks) and efficacy achieved, consider 50% reduction in daily dose and monitor for continued efficacy."
Likely Intermediate Metabolizer	Per CPIC (A): "Initiate standard starting daily dose. For chronic therapy (>12 weeks) and efficacy achieved, consider 50% reduction in daily dose and monitor for continued efficacy."
Likely Poor Metabolizer	Per CPIC (A): "Initiate standard starting daily dose. For chronic therapy (>12 weeks) and efficacy achieved, consider 50% reduction in daily dose and monitor for continued efficacy."
Normal Metabolizer	Per CPIC (A): "Initiate standard starting daily dose. Consider increasing dose by 50-100% for the treatment of H. pylori infection and erosive esophagitis. Daily dose may be given in divided doses. Monitor for efficacy."
Poor Metabolizer	Per CPIC (A): "Initiate standard starting daily dose. For chronic therapy (>12 weeks) and efficacy achieved, consider 50% reduction in daily dose and monitor for continued efficacy."
Rapid Metabolizer	Per CPIC (A): "Initiate standard starting daily dose. Consider increasing dose by 50-100% for the treatment of H. pylori infection and erosive esophagitis. Daily dose may be given in divided doses. Monitor for efficacy."
Ultrarapid Metabolizer	Per CPIC (A): "Increase starting daily dose by 100%. Daily dose may be given in divided doses. Monitor for efficacy."

escitalopram

Some antidepressants, including escitalopram and citalopram, are partly cleared by CYP2C19. In poor metabolizers the levels can run higher, which is why guidance points to a slower dose increase and watching for side effects.

If your result is	Published CPIC guidance
Intermediate Metabolizer	Per CPIC (A): "Initiate therapy with recommended starting dose. Consider a slower titration schedule and lower maintenance dose than normal metabolizers."
Likely Intermediate Metabolizer	Per CPIC (A): "Initiate therapy with recommended starting dose. Consider a slower titration schedule and lower maintenance dose than normal metabolizers."
Likely Poor Metabolizer	Per CPIC (A): "Consider a clinically appropriate antidepressant not predominantly metabolized by CYP2C19. If citalopram or escitalopram are clinically appropriate, consider a lower starting dose, slower titration schedule and 50% reduction of the standard maintenance dose as compared to normal metabolizers."
Normal Metabolizer	Per CPIC (A): "Initiate therapy with recommended starting dose"
Poor Metabolizer	Per CPIC (A): "Consider a clinically appropriate antidepressant not predominantly metabolized by CYP2C19. If citalopram or escitalopram are clinically appropriate, consider a lower starting dose, slower titration schedule and 50% reduction of the standard maintenance dose as compared to normal metabolizers."
Rapid Metabolizer	Per CPIC (A): "Initiate therapy with recommended starting dose. If patient does not adequately respond to recommended maintenance dosing, consider titrating to a higher maintenance dose or switching to a clinically appropriate alternative antidepressant not predominantly metabolized by CYP2C19."
Ultrarapid Metabolizer	Per CPIC (A): "Consider a clinically appropriate alternative antidepressant not predominantly metabolized by CYP2C19. If citalopram or escitalopram are clinically appropriate, and adequate efficacy is not achieved at standard maintenance dosing, consider titrating to a higher maintenance dose."

Is this in your raw DNA file?

Consumer kits from 23andMe, AncestryDNA and others are genotyping chips. They read a few hundred thousand chosen positions, and which ones depends on the vendor and the chip version. The report needs three of them to call CYP2C19: the *2, *3 and *17 positions. Of ten consumer chip exports we measured, the three from 23andMe and a 2024 AncestryDNA export carried all three, and the other six all missed *17. Rarer and structural variants are not part of a chip reading. When your file does not settle all three positions, we report CYP2C19 as Indeterminate instead of guessing, unless both copies already carry a variant the chip did read.

A genotyping chip reads only a few hundred thousand pre-selected positions, about 0.02% of your genome, chosen for common variation. It does not sequence the rest, so it cannot find the rare or novel pathogenic variants the clinical and carrier modules look for: a “no finding” from chip data means “this chip never looked”, far more so than with whole-genome sequencing. Chip data suits common-variant traits, pharmacogenomic tag SNPs, and haplogroup ancestry, not clinical or carrier screening.