Omeprazole
Brand names: Losec
Omeprazole is a proton pump inhibitor used to treat gastro-oesophageal reflux disease, peptic ulcer disease, as part of Helicobacter pylori eradication, and for gastroprotection in patients taking NSAIDs.
Adult dose
Dose adjustments
Dose adjustment is not needed in patients with impaired renal function.
Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.
Contraindications
- Hypersensitivity to the active substance, to substituted benzimidazoles, or to any of the excipients
- Must not be used concomitantly with nelfinavir
Side effects
- Common (1-10%): headache
- Common: abdominal pain, constipation, diarrhoea, flatulence, nausea/vomiting, benign fundic gland polyps
- Uncommon: dizziness, paraesthesia, somnolence, insomnia, vertigo; increased liver enzymes
- Uncommon: dermatitis, pruritus, rash, urticaria; fracture of the hip, wrist or spine
- Not known: hypomagnesaemia (may result in hypocalcaemia and be associated with hypokalaemia); microscopic colitis
- Rare/very rare: severe cutaneous adverse reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis, DRESS, AGEP), hypersensitivity reactions including anaphylaxis, tubulointerstitial nephritis, hepatitis, agranulocytosis/pancytopenia
Interactions
- Nelfinavir — concomitant use is contraindicated (§4.3)
- Atazanavir — co-administration with proton pump inhibitors is not recommended; if judged unavoidable, close clinical monitoring with atazanavir increased to 400 mg plus ritonavir 100 mg, and omeprazole 20 mg should not be exceeded (§4.4)
- Clopidogrel — an interaction is observed with omeprazole; clinical relevance uncertain, but as a precaution concomitant use should be discouraged (§4.4)
- Omeprazole is a CYP2C19 inhibitor — consider the potential for interactions with drugs metabolised through CYP2C19 when starting or stopping omeprazole (§4.4)
- Digoxin and drugs that may cause hypomagnesaemia (e.g. diuretics) — consider measuring magnesium levels before and periodically during prolonged PPI treatment (§4.4)
- May reduce absorption of vitamin B12 (cyanocobalamin) due to hypo- or achlorhydria on long-term therapy (§4.4)
Clinical monograph
How it works
It irreversibly inhibits the gastric H+/K+-ATPase (proton pump) in parietal cells, markedly reducing basal and stimulated gastric acid secretion.
Prescribing in practice
- It can mask the symptoms of gastric cancer, so investigate alarm features such as unexplained weight loss, dysphagia or gastrointestinal bleeding before attributing them to reflux.
- It reduces the antiplatelet effect of clopidogrel and may lower magnesium with long-term use, so review the indication and duration periodically.
- Prolonged use is associated with increased risk of fractures and gastrointestinal infections, so use the lowest effective dose for the shortest necessary period.
Monitoring
Review the ongoing need for treatment regularly, and consider checking magnesium with long-term use or when combined with other magnesium-lowering drugs.
Counselling the patient
- Take it before food, usually in the morning, for best effect.
- Tell your doctor if you develop difficulty swallowing, weight loss or black stools.
- Do not stop or continue long term without review, as long-term use carries some risks.
Evidence & guidelines
Proton pump inhibitors including omeprazole are first-line acid-suppression therapy in NICE dyspepsia and gastro-oesophageal reflux guidance.
Reference: NICE NG184 (Gastro-oesophageal Reflux Disease); MHRA Drug Safety Update (Hypomagnesaemia); Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).
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