The (R)-enantiomer of omeprazole is metabolized exclusively by the enzyme CYP2C19, which is expressed in very low amounts by 3% of the population. Treated with a normal dose of the enantiomeric mixture, these persons will experience blood levels five-times higher than those with normal CYP2C19 production. In contrast, esomeprazole is metabolized by both CYP2C19 and CYP3A4, providing less-variable drug exposure. While omeprazole is approved only at doses of up to 20 mg for the treatment of gastroesophageal reflux, esomeprazole is approved for doses up to 40 mg.