People who experience any of the symptoms while taking the medication are urged by the NHS to see a doctor
Charlotte Smith Senior Lifestyle Content Editor
13:15, 18 Feb 2026Updated 14:36, 18 Feb 2026
People told “see a doctor” if they are suffering from a “serious” side effect (stock image)(Image: Getty)
Patients who have been prescribed omeprazole by their doctor to help treat heartburn, acid reflux and other stomach-related issues should be aware of ‘serious’ side effects, especially if they have been taking the medication for a long time. According to the latest NHS guidance, patients taking the medication are urged to see their GP if they experience extreme fatigue.
Omeprazole is a widely prescribed medicine in the UK, with about 36 million prescriptions given out each year for acid reflux and stomach ulcers. It ranks as one of the top three most prescribed drugs in the country, and data shows that around 15% of people in the UK might be affected by or use this medication.
Omeprazole is a type of proton pump inhibitor (PPI), which works by reducing the amount of acid you have in your stomach. There are some low-dose versions of the medication available over the counter at pharmacies, but stronger options are prescription-only. Some people may need to take omeprazole for several months or longer, depending on their individual circumstances, and it shouldn’t be taken for any longer than necessary.
Using Omeprazole for a long time can increase the risk of a vitamin B12 deficiency because less stomach acid makes it harder for your body to absorb nutrients. The NHS recommends checking your B12 levels if you’ve been using it for more than two to three years.
Some signs of this deficiency are feeling very tired, having weak muscles, and experiencing tingling sensations. The latest advice on the NHS website states: “Omeprazole can sometimes cause serious side effects, but most of them are rare.
Vitamin B12 or folate deficiency anaemia can cause a wide range of symptoms – including fatigue
“Possible serious side effects of omeprazole include having a very low level of vitamin B12, which can cause symptoms such as tiredness, dizziness, or muscle twitches or weakness.” The health service also urges: “Stomach acid-lowering drugs should ideally be discontinued if possible, and we would advise you to book an appointment with the doctor or clinical pharmacist to discuss this.”
Vitamin B12 is important for keeping our nervous system and blood cells in good shape. If you don’t have enough vitamin B12, it can lead to anaemia, which makes you feel tired, low on energy, short of breath, dizzy, and can give you headaches.
You can also have low vitamin B12 without having anaemia. This might lead to problems like tingling sensations, blurry vision, sores in your mouth, a sore tongue, trouble remembering things, and feeling weak in your muscles.
The NHS advises people to see their GP if they experience any of these symptoms (stock image)
The NHS advises: “See a GP if you’re experiencing symptoms of vitamin B12 or folate deficiency anaemia. These conditions can often be diagnosed based on your symptoms and the results of a blood test.
“It’s important for vitamin B12 or folate deficiency anaemia to be diagnosed and treated as soon as possible. Although many of the symptoms improve with treatment, some problems caused by the condition can be irreversible if left untreated. The longer the condition goes untreated, the higher the chance of permanent damage.”
On the NHS website, it also instructs people to “speak to a pharmacist or doctor if you have any side effects that bother you or do not go away. They may be able to give you advice about how you can reduce the side effects. Keep taking your medicine unless you’re advised not to.”
While extreme fatigue can be caused by many factors besides medication, it is important to get checked so doctors can investigate the root cause and develop a treatment plan.
Other side effects of omeprazole
“Like all medicines, omeprazole can cause side effects, but not everyone gets them,” the NHS says. Common side effects of omeprazole include:
a headachestomach painfeeling sick (nausea) or being sick (vomiting)constipation or diarrhoeafarting more than usual
Besides low vitamin B12 levels, omeprazole can sometimes cause other “serious” side effects, “but most of them are rare”, the health service adds. Possible serious side effects of omeprazole include:
weakened bones, which can break more easily if you have a fall or injuryan allergic reaction (anaphylaxis), which can cause symptoms such as a swollen throat or tongue, a raised, itchy rash or difficulty breathinga type of lupus that can cause rashes (particularly on parts of the body exposed to the sun) and joint pain
The chance of getting serious side effects can be higher if you take omeprazole for a long time (a year or more).