Omeprazole

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Omeprazole on Prescriptsy

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Omeprazole is a proton pump inhibitor (PPI) used to reduce the production of stomach acid.

It is one of the most widely prescribed medicines in the United Kingdom, with over 50 million prescriptions dispensed annually across the NHS.

Omeprazole is used to treat gastro-oesophageal reflux disease (GORD), peptic ulcers (stomach and duodenal), Zollinger-Ellison syndrome, NSAID-associated ulcers, and as part of combination therapy to eradicate the bacterium Helicobacter pylori.

It is available as gastro-resistant capsules (10 mg, 20 mg, and 40 mg), dispersible tablets, and as an intravenous injection for hospital use.

Omeprazole is a prescription-only medicine (POM) at prescription-strength doses, with 10 mg and 20 mg packs also available as pharmacy (P) medicines for short-term heartburn relief.

Acid-related disorders of the upper gastrointestinal tract are among the most common reasons for GP consultations and hospital referrals in the UK.

GORD alone affects approximately 10% to 20% of the adult population, causing symptoms such as heartburn, acid regurgitation, chest discomfort, and, in some cases, oesophageal damage (oesophagitis, Barrett's oesophagus).

Peptic ulcer disease, though declining in prevalence due to improved H. pylori eradication and reduced NSAID-related harm, remains clinically significant.

This page provides a comprehensive clinical overview of omeprazole, covering how it works, dosage guidance, side effects, safety warnings, and how to obtain it in the United Kingdom.

Important safety information about omeprazole

Before reading further, please note these essential safety points.

• Use the lowest effective dose for the shortest duration necessary. Your prescriber should review ongoing PPI therapy at least annually.
• If you have new or worsening digestive symptoms, particularly with difficulty swallowing, weight loss, vomiting, or blood in your vomit or stools, seek urgent medical attention. These symptoms require investigation before starting a PPI.
• Do not take omeprazole with clopidogrel unless your prescriber has specifically assessed the interaction. Alternative PPIs may be more suitable.
• Long-term PPI use has been associated with hypomagnesaemia, fracture risk, vitamin B12 deficiency, and Clostridium difficile infection. Benefits usually outweigh risks, but regular review is important.
• If stopping omeprazole after prolonged use, reduce the dose gradually over 2 to 4 weeks to avoid rebound acid symptoms.

Understanding acid-related disorders

The stomach produces hydrochloric acid through parietal cells in the gastric mucosa.

This acid is essential for digestion (activating pepsinogen to pepsin, denaturing proteins) and defence against ingested pathogens.

However, when acid is produced in excess, or when the mucosal defences are impaired, the acid can damage the lining of the oesophagus, stomach, or duodenum, leading to conditions such as reflux oesophagitis, gastric ulcer, and duodenal ulcer.

Gastro-oesophageal reflux disease occurs when stomach acid frequently flows back into the oesophagus.

The lower oesophageal sphincter (LOS), a ring of muscle at the junction of the oesophagus and stomach, normally prevents reflux.

When the LOS is weakened or relaxes inappropriately, acid can enter the oesophagus, causing the burning sensation of heartburn, acid regurgitation, and, over time, inflammation and damage to the oesophageal lining.

Risk factors for GORD include obesity, hiatus hernia, pregnancy, smoking, and certain foods and drinks (fatty foods, chocolate, caffeine, alcohol, citrus, and tomatoes).

Peptic ulcers are breaks in the lining of the stomach or duodenum.

The two main causes are infection with Helicobacter pylori (a bacterium that colonises the gastric mucosa and weakens the mucosal defence) and the use of non-steroidal anti-inflammatory drugs (NSAIDs), which inhibit prostaglandin production and reduce mucosal blood flow and bicarbonate secretion.

Symptoms include burning epigastric pain, nausea, bloating, and, in severe cases, haematemesis (vomiting blood) or melaena (black tarry stools indicating upper GI bleeding).

How omeprazole works: mechanism of action

Omeprazole belongs to the class of medicines known as proton pump inhibitors.

The proton pump (H+/K+ ATPase) is an enzyme located on the apical membrane of gastric parietal cells.

It is the final common pathway for hydrochloric acid secretion into the stomach lumen.

All stimulatory signals for acid secretion (histamine via H2 receptors, acetylcholine via muscarinic M3 receptors, gastrin via CCK-B receptors) converge on the proton pump.

By inhibiting this pump, omeprazole suppresses acid secretion regardless of the stimulus.

Omeprazole is a pro-drug. After oral absorption, it enters the bloodstream and is delivered to the parietal cells.

It accumulates in the acidic secretory canaliculi of the parietal cells, where the pH is approximately 1.

In this highly acidic environment, omeprazole undergoes protonation and is converted to its active sulphenamide form.

The sulphenamide then forms a covalent (irreversible) disulphide bond with cysteine residues on the proton pump, permanently inactivating the enzyme.

Because the binding is irreversible, acid secretion can only resume when the parietal cell synthesises new proton pump molecules, a process that takes approximately 18 to 24 hours.

This is why omeprazole's duration of acid suppression (approximately 24 hours) far exceeds its plasma half-life (approximately 1 hour).

Maximum acid suppression is not achieved with the first dose.

Because omeprazole can only inhibit proton pumps that are actively secreting acid at the time of dosing, and parietal cells continuously synthesise new pumps, it takes 3 to 5 days of daily dosing to reach a steady state of acid suppression.

This is why patients may not experience full symptom relief for several days after starting treatment.

Clinical evidence and UK prescribing guidance

Proton pump inhibitors have been the subject of extensive clinical research since omeprazole's introduction in 1989, and they represent one of the most evidence-based classes of medicine in clinical use.

Omeprazole has been shown in numerous randomised controlled trials to be superior to H2 receptor antagonists (ranitidine, famotidine) for healing oesophagitis, gastric and duodenal ulcers, and maintaining remission.

Healing rates for duodenal ulcer at 4 weeks are approximately 90% to 95% with omeprazole 20 mg daily, compared with 70% to 80% with ranitidine 300 mg daily.

NICE guideline CG184 (gastro-oesophageal reflux disease and dyspepsia in adults) recommends a step-down approach: start with a full-dose PPI for 4 to 8 weeks, then step down to the lowest dose that controls symptoms, and consider stopping if symptoms resolve.

For uninvestigated dyspepsia, NICE recommends testing for H. pylori before starting a PPI. If H. pylori is positive, eradication therapy should be given first.

For NSAID-associated ulcers, co-prescription of a PPI is recommended for the duration of NSAID therapy in patients at increased GI risk (aged 65 and over, history of peptic ulcer, concomitant anticoagulant or corticosteroid use).

The BNF provides detailed prescribing information for omeprazole, including dose adjustments for children, hepatic impairment, and combination regimens for H. pylori eradication. Omeprazole is the most commonly prescribed PPI in the UK, followed by lansoprazole and pantoprazole.

Omeprazole compared with other PPIs

Five PPIs are available in the UK: omeprazole, esomeprazole, lansoprazole, pantoprazole, and rabeprazole. All share the same mechanism of action and have similar clinical efficacy for most indications.

Esomeprazole is the S-enantiomer of omeprazole and has marginally higher bioavailability, but head-to-head trials show only small clinical differences.

Lansoprazole and pantoprazole have a lower potential for CYP2C19-mediated drug interactions than omeprazole, making them preferred alternatives when a PPI is needed alongside clopidogrel.

Rabeprazole is metabolised predominantly by a non-enzymatic pathway and has the lowest interaction potential among PPIs.

Omeprazole is available as a generic medicine and is the least expensive PPI in the UK. NICE and NHS formularies generally recommend omeprazole or lansoprazole as first-line PPIs.

The choice between PPIs is influenced by cost, drug interactions, and individual patient response.

Dosage and administration

Omeprazole capsules should be taken once daily in the morning, before breakfast, swallowed whole with a glass of water.

The enteric coating on the granules inside the capsule protects the active ingredient from degradation in the stomach.

Chewing or crushing the capsule destroys this protection and may reduce efficacy. For patients who cannot swallow capsules, MUPS (multiple unit pellet system) dispersible tablets are available.

These can be dissolved in water or slightly acidic fruit juice, stirred, and drunk immediately.

For GORD, the standard dose is 20 mg once daily for 4 to 8 weeks. Maintenance therapy is 10 to 20 mg daily.

For duodenal ulcer, 20 mg once daily for 4 weeks. For gastric ulcer, 20 mg once daily for 8 weeks. For H.

pylori eradication, 20 mg twice daily with two antibiotics for 7 days. For Zollinger-Ellison syndrome, 60 mg once daily initially, adjusted according to response.

For NSAID prophylaxis, 20 mg once daily.

In children, the dose is weight-based. In severe hepatic impairment, the dose should not exceed 20 mg daily. No dose adjustment is needed for renal impairment.

Side effects of omeprazole

Common side effects

Headache, nausea, abdominal pain, diarrhoea, constipation, and flatulence are the most frequently reported side effects and are generally mild. Dizziness, insomnia, and dry mouth occur less commonly.

Long-term risks

Hypomagnesaemia can develop after 3 months or more of PPI use and may cause muscle cramps, tremor, palpitations, or seizures.

The MHRA advises checking magnesium levels in patients on long-term PPIs, especially those also taking digoxin or diuretics.

Fracture risk is modestly increased with long-term PPI use, particularly hip, wrist, and spine fractures in older patients.

Vitamin B12 deficiency may occur after 3 or more years of continuous PPI use.

Clostridium difficile-associated diarrhoea is more common in PPI users, particularly in hospital settings and elderly patients. Rebound acid hypersecretion may occur on abrupt discontinuation after prolonged therapy.

Drug interactions

Omeprazole inhibits CYP2C19 and may increase plasma levels of diazepam, phenytoin, and cilostazol.

The interaction with clopidogrel is clinically significant: omeprazole reduces the antiplatelet effect of clopidogrel by approximately 40% to 45%. The MHRA advises avoiding this combination.

Omeprazole may reduce absorption of ketoconazole, itraconazole, and atazanavir. Monitor INR when using omeprazole with warfarin. High-dose methotrexate may accumulate when given with PPIs.

When to seek medical advice

Seek urgent medical attention if you vomit blood, pass black tarry stools, develop difficulty swallowing, experience unexplained weight loss, or have persistent vomiting.

These may indicate a serious condition that requires investigation.

Contact your GP or call NHS 111 for persistent or worsening symptoms despite PPI therapy, new-onset symptoms in patients over 55, or signs of hypomagnesaemia (muscle cramps, tremor, palpitations).

Report suspected adverse reactions to the MHRA at yellowcard.mhra.gov.uk .

Warnings and precautions

Excluding serious pathology

Omeprazole may mask the symptoms of gastric cancer. Before starting treatment, particularly in patients aged 55 and over with new-onset dyspepsia, alarm symptoms should be excluded.

NICE NG12 recommends urgent 2-week-wait referral for upper GI endoscopy in patients with dysphagia, unexplained weight loss, upper abdominal mass, or other red-flag features.

Annual review

NICE CG184 recommends reviewing PPI therapy at least annually. Many patients take PPIs for longer than necessary.

If the original indication has resolved, a trial of dose reduction or stopping the PPI (with step-down over 2 to 4 weeks) should be considered.

Lifestyle measures (weight loss, raising the head of the bed, avoiding trigger foods, not eating late at night) may be sufficient for mild GORD.

Clopidogrel interaction

The MHRA has issued specific advice: omeprazole should not be used with clopidogrel unless there is a clear clinical need and no suitable alternative PPI. Lansoprazole or pantoprazole are preferred if a PPI is required alongside clopidogrel.

Osteoporosis and fracture

Patients at risk of osteoporosis (elderly, postmenopausal women, those on corticosteroids) should receive appropriate treatment (calcium, vitamin D, weight-bearing exercise) if long-term PPI therapy is required.

How to get omeprazole in the UK

Omeprazole is available on NHS prescription from your GP or an authorised online prescriber registered with the General Pharmaceutical Council (GPhC).

The standard NHS prescription charge in England is 9.90 pounds per item; prescriptions are free in Scotland, Wales, and Northern Ireland.

Omeprazole 10 mg and 20 mg are also available from pharmacies without a prescription for short-term relief of reflux symptoms in adults aged 18 and over.

Sources

• Omeprazole Capsules, Summary of Product Characteristics (EMC)
• Omeprazole, British National Formulary (BNF)
• NICE CG184: Gastro-oesophageal reflux disease and dyspepsia in adults
• Omeprazole, NHS
• MHRA Yellow Card Scheme