[13] Common side effects include nausea, vomiting, headaches, abdominal pain, and increased intestinal gas.

[32] Since their introduction, proton-pump inhibitors (PPIs, especially omeprazole) have also been associated with several cases of acute interstitial nephritis,[33] an inflammation of the kidneys that often occurs as an adverse drug reaction.

[35] A review article in U.S. Pharmacist in 2013 states that long-term use of PPIs is associated with decreased calcium absorption (causing increased risk of osteoporosis and fractures), decreased magnesium absorption (causing electrolyte disturbances), and increased risk of certain infections such as C. difficile and community-acquired pneumonia.

[14] Epidemiological data do not show an increased risk of major birth defects after maternal use of omeprazole during pregnancy.

[40] Although still controversial,[41] this may increase the risk of stroke or heart attack in people taking clopidogrel to prevent these events.

[43][44] Almost all benzodiazepines are metabolised by the CYP3A4 and CYP2D6 pathways, and inhibition of these enzymes results in a higher area under the curve (i.e., the total effect over time of a given dose).

[47] St. John's wort (Hypericum perforatum) and Ginkgo biloba significantly reduce plasma concentrations of omeprazole through induction of CYP3A4 and CYP2C19.

[50] Omeprazole irreversibly blocks the enzyme system on parietal cells that is needed for the secretion of gastric acid.

It suppresses stomach acid secretion by specific inhibition of the H+/K+-ATPase system found at the secretory surface of gastric parietal cells.

The inhibitory effect of omeprazole on acid secretion will plateau after 4 days of repeated daily dosing.

[14] Identified metabolites are the sulfone, the sulfide, and hydroxy-omeprazole, which exert no significant effect on acid secretion.

About 77% of an orally given dose is excreted as metabolites in the urine, and the remainder is found in the feces, primarily originating from bile secretion.

[citation needed] AstraZeneca also developed esomeprazole (Nexium) which is a eutomer, purely the (S)-enantiomer, rather than a racemate like omeprazole.

[60] 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.

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).

Although the solution by means of two coating was obvious, the patent was found valid, because the source of the problem was non-obvious and was discovered by the patentee.

[67] In September 2023, AstraZeneca announced it would pay $425 million to settle product liability litigations against Prilosec in the United States.