Omeprazole, sold under the brand names Prilosec and Losec, among others, is a medication used in the treatment of gastroesophageal reflux disease (GERD), peptic ulcer disease, and Zollinger–Ellison syndrome. It is also used to prevent upper gastrointestinal bleeding in people who are at high risk. Omeprazole is a proton-pump inhibitor (PPI) and its effectiveness is similar to other PPIs. It can be taken by mouth or by injection into a vein.Common side effects include nausea, vomiting, headaches, abdominal pain, and increased intestinal gas. Serious side effects may include Clostridium difficile colitis, an increased risk of pneumonia, an increased risk of bone fractures, and the potential of masking stomach cancer. It is unclear if it is safe for use in pregnancy. It works by blocking the release of stomach acid.Omeprazole was patented in 1978, and approved for medical use in 1988. It is on the World Health Organizations List of Essential Medicines. It is available as a generic medication. In 2020, it was the eighth most commonly prescribed medication in the United States, with more than 56 million prescriptions. It is also available without a prescription in the United States. Medical uses
Omeprazole can be used in the treatment of gastroesophageal reflux disease (GERD), peptic ulcers, erosive esophagitis, Zollinger–Ellison syndrome, and eosinophilic esophagitis. Peptic ulcers
Peptic ulcers may be treated with omeprazole. Treatment of a Helicobacter pylori infection can be completed by taking a triple therapy combination of omeprazole, amoxicillin, and clarithromycin for 7–14 days. Amoxicillin may be replaced with metronidazole in patients who are allergic to penicillin.
Adverse effects
Adverse effects occurring in at least 1% of people include:
Central nervous system: headache (7%), dizziness (2%)
Respiratory: upper respiratory tract infection (2%), cough (1%)
Gastrointestinal: abdominal pain (5%), diarrhea (4%), nausea (4%), vomiting (3%), flatulence (3%), acid regurgitation (2%), constipation (2%)
Neuromuscular and skeletal: back pain (1%), weakness (1%)
Dermatologic: rash (2%)Other concerns related to adverse effects are:

Recurrence of Clostridium difficile associated diarrhea
Osteoporosis-related fractures
HypomagnesemiaConcern has been expressed regarding vitamin B12 and iron malabsorption, but effects seem to be insignificant, especially when supplement therapy is provided.Since their introduction, proton-pump inhibitors (PPIs, especially omeprazole) have also been associated with several cases of acute interstitial nephritis, an inflammation of the kidneys that often occurs as an adverse drug reaction. Long-term use
Long-term use of PPIs is strongly associated with the development of benign polyps from fundic glands (which is distinct from fundic gland polyposis); these polyps do not cause cancer and resolve when PPIs are discontinued. No association is seen between PPI use and cancer, but use of PPIs may mask gastric cancers or other serious gastric problems and physicians should be aware of this effect.There is a possible association between long term use and dementia which requires further study to confirm.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. They hypothesize that this is due to decreased stomach acid production.