Omeprazole undergoes a chiral shift in vivo which converts the inactive (R)-enantiomer to the active (S)-enantiomer, doubling the concentration of the active form. This chiral shift is accomplished by the CYP2C19 isozyme of cytochrome P450, which is not found equally in all human populations. Those who do not metabolize the drug effectively are called "poor metabolizers". The proportion of the poor metabolizer phenotype varies widely between populations, from 2.0 to 2.5% in African Americans and white Americans to >20% in Asians. Several pharmacogenomics studies have suggested that PPI treatment should be tailored according to CYP2C19 metabolism status. Measurement in body fluids
Omeprazole may be quantified in plasma or serum to monitor therapy or to confirm a diagnosis of poisoning in hospitalized patients. Plasma omeprazole concentrations are usually in a range of 0.2–1.2 mg/L in persons receiving the drug therapeutically by the oral route and 1–6 mg/L in people with acute overdose. Enantiomeric chromatographic methods are available to distinguish esomeprazole from racemic omeprazole. History
Omeprazole was first made in 1979 by Swedish AB Hässle, part of Astra AB. It was the first of the proton pump inhibitors (PPI). Astra AB, now AstraZeneca, launched it as an ulcer medicine under the name Losec in Sweden. It was first sold in the United States in 1989 under the brand name Losec. In 1990, at the request of the U.S. Food and Drug Administration, the brand name Losec was changed to Prilosec to avoid confusion with the diuretic Lasix (furosemide).
Pregnancy and breastfeeding
The safety of using omeprazole has not been established in pregnant or breastfeeding women. Epidemiological data do not show an increased risk of major birth defects after maternal use of omeprazole during pregnancy.No clinical trials have deeply evaluated the potential consequences of the use of omeprazole in breastfeeding. However, the pharmacokinetics of the omeprazole molecule strongly suggest the safety of omeprazole use during breastfeeding:

Omeprazole has a high plasma protein binding rate (95%), indicating that a little amount of drug is transferred to the milk duct during breast milk formation. Omeprazole needs to be administered in an enteric-coated formulation due to its rapid degradation in the acidic conditions of the stomach. This suggests that most of the free molecules ingested by the infant are likely degraded before being absorbed.Omeprazole at normal doses is likely safe during breastfeeding. Interactions
Important drug interactions are rare. However, the most significant major drug interaction concern is the decreased activation of clopidogrel when taken together with omeprazole. Although still controversial, this may increase the risk of stroke or heart attack in people taking clopidogrel to prevent these events. This interaction is possible because omeprazole is an inhibitor of the enzymes CYP2C19 and CYP3A4. Clopidogrel is an inactive prodrug that partially depends on CYP2C19 for conversion to its active form.