Omeprazole: uses, side effects and cautions

Omeprazole belongs to the class of medicines known as proton pump inhibitors. Its job is simple to state: it turns down acid production in the lining of the stomach. Less acid means less irritation of the stomach wall and food pipe, which is why it is a first-line choice when acid is doing damage.

According to the NHS medicines pages, the conditions it treats or prevents are stomach ulcers, heartburn, gastro-oesophageal reflux disease (GORD) and infection with Helicobacter pylori, a stomach bacterium that is cleared using omeprazole together with antibiotics. A separate prescribed form is used for children whose reflux is severe enough to cause discomfort.

It comes as tablets, capsules and a liquid. Some tablets are designed to dissolve in water, which can help people who struggle to swallow.

The stomach lining contains tiny pumps that push acid into the stomach. Omeprazole blocks those pumps, so the stomach simply produces less acid. That is why it acts differently from a liquid antacid, which neutralises acid that is already there.

The NHS page does not give a time to first effect for omeprazole. In practice, people on prescription courses are told to keep taking it regularly rather than only when pain strikes, because the benefit builds up with consistent use. Your prescriber or the leaflet in the box will say how long to expect to wait.

Omeprazole is usually taken once or twice a day, at the same times each day, with or without food. Once-daily doses are normally taken in the morning; twice-daily doses are taken morning and evening. The right amount depends on the reason it was prescribed and the form, so follow your prescriber and the patient leaflet rather than a figure from a website.

Gastro-resistant tablets and capsules have a coating that protects the medicine from stomach acid, so they should be swallowed with water and never chewed or crushed.

• Some people need it for a few days, others for weeks, months or years, depending on the condition.

• If you bought a non-prescription pack, the NHS says not to use it for longer than 2 weeks before seeing a doctor.

• Missed a dose? Take it when you remember unless the next one is nearly due, in which case skip it. Never take two to make up.

• An extra dose is unlikely to cause harm, but ask a pharmacist or NHS 111 for advice if you have taken too much.

Omeprazole is not suitable for everyone. The NHS lists a previous allergic reaction or serious side effect to omeprazole or another PPI, as well as kidney or liver disease and osteoporosis, as reasons to check first with a doctor or pharmacist.

Tell the prescriber about every other medicine you take and about any tests planned. Omeprazole can affect some blood tests, tumour screening and gastroscopy results, and you may be asked to stop it for a few days or weeks beforehand. Do not make that change on your own; your clinician will say if it applies.

Common effects include headache, stomach pain, feeling or being sick, constipation, diarrhoea and wind. If they bother you or do not settle, speak to a pharmacist or doctor.

Rarer but important problems are mostly linked with long-term use. They include very low vitamin B12 (tiredness, dizziness and weakness), weaker bones with a higher chance of fractures, and a lupus-type reaction with rashes and joint pain. Because omeprazole can also mask the symptoms of serious stomach conditions, new alarm symptoms should always be checked rather than assumed to be reflux.

• Call NHS 111 if you think you may be having a serious side effect or have taken more than advised.

• Call 999 for signs of a severe allergic reaction, such as a swollen throat or tongue, a spreading itchy rash or difficulty breathing. Do not drive yourself to hospital.

• Suspected side effects can be reported through the Yellow Card scheme.

The NHS says omeprazole can usually be taken in pregnancy and while breastfeeding, but it asks you to check with a pharmacist or doctor first. It is rare for breastfed babies to have side effects from a mother's medicine.

Medicines the NHS names as interacting include other acid reducers such as lansoprazole and famotidine, digoxin, the blood-thinning and clot-preventing medicines warfarin and clopidogrel, diazepam, phenytoin, and the HIV medicines nelfinavir and atazanavir. The herbal remedy St John's wort is also listed. Do not start or stop any of these on the strength of a website; ask your pharmacist to check your full list.

Alcohol does not stop omeprazole from working, but it tends to increase acid, so the NHS suggests it is best avoided. You can eat normally, and you do not need to time doses around meals.

Omeprazole does not usually affect driving or cycling. If you feel dizzy or your vision is blurred, do not drive until it passes.

Anyone taking it for more than a year should expect periodic review of whether the dose is still needed. Reducing or stopping after long use is something to plan with the prescriber rather than do abruptly, because acid symptoms can bounce back.

Omeprazole is a prescription-only medicine in most forms, with some lower-strength products available from pharmacies and shops for adults. Brands the NHS lists are Losec, Losec MUPS and Pyrocalm.

Other PPIs include lansoprazole, esomeprazole and pantoprazole, which work in a similar way and are covered in their own guides. Lifestyle measures such as smaller meals, avoiding late eating and raising the head of the bed can also reduce reflux. What suits you is a decision for your prescriber.