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Side effects of omeprazole - Brand names: Losec, Losec MUPS, Pyrocalm

Common side effects

These common side effects of omeprazole happen in more than 1 in 100 people. There are things you can do to help cope with them.


Make sure you rest and drink plenty of fluids, but it’s best to not to drink alcohol. Headaches usually go away after the first week. Ask your pharmacist to recommend a painkiller, or talk to your doctor if they last more than a week or are severe.

Feeling sick (nausea)

Try taking omeprazole with or after a meal or snack. It may also help to not eat rich or spicy food.

Being sick (vomiting) or diarrhoea

Drink plenty of water by having small, frequent sips. This can help to avoid dehydration. Signs of dehydration include peeing less than usual or having strong-smelling pee. Do not take any other medicines to treat diarrhoea or vomiting without speaking to a pharmacist or doctor. If diarrhoea doesn’t get better, talk to your doctor.

If you take contraceptive pills and you’re being sick or have severe diarrhoea for more than 24 hours, your contraception may not protect you from pregnancy. Check the pill packet for advice

Stomach pain

Try to rest and relax. It can help to eat and drink slowly and have smaller and more frequent meals. Putting a heat pad or covered hot water bottle on your stomach may also help. If you are in a lot of pain, speak to your pharmacist or doctor.


Get more fibre into your diet, such as fresh fruit and vegetables and cereals, and drink plenty of water. Try to exercise, for example, by going for a daily walk or run. If this doesn't help, talk to your pharmacist or doctor.

Farting (flatulence)

Try to avoid foods that cause farting (flatulence), like pulses, lentils, beans and onions. It might also help to eat smaller meals more frequently, eat and drink slowly, and exercise regularly. Some pharmacy remedies, such as simethicone, may also help with this.

If this advice does not help and any of these side effects continue to bother you, tell your doctor or pharmacist.

Serious side effects

Serious side effects are rare and happen in less than 1 in 1,000 people.

Call a doctor or contact 111 straight away if:

  • your skin becomes more yellow (although this may be less obvious on brown or black skin), your pee becomes darker and you feel more tired – these can be signs of liver problems
  • you get joint pain along with a red skin rash, especially on parts of your body exposed to the sun, such as your arms, cheeks and nose – these can be signs of a rare condition called subacute cutaneous lupus erythematosus. This can happen even if you have been taking omeprazole for a long time
  • severe or persistent diarrhoea – this can be a sign of an inflamed bowel

Go to or call 111.

If you need advice for a child under the age of 5 years, call 111.

Serious allergic reaction

In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to omeprazole.

Immediate action required: Call 999 or go to A&E now if:

  • you get a skin rash that may include itchy, red, swollen, blistered or peeling skin
  • you're wheezing
  • you get tightness in the chest or throat
  • you have trouble breathing or talking
  • your mouth, face, lips, tongue or throat start swelling

You could be having a serious allergic reaction and may need immediate treatment in hospital.

Long-term side effects

If you take omeprazole for more than 3 months, the levels of magnesium in your blood may fall.

Low magnesium can make you feel tired, confused, dizzy and cause muscle twitches, shakiness and an irregular heartbeat. If you get any of these symptoms, tell your doctor.

Taking omeprazole for more than a year may increase your chances of certain side effects, including:

  • bone fractures
  • gut infections
  • vitamin B12 deficiency – symptoms include feeling very tired, a sore and red tongue, mouth ulcers and pins and needles

If you take omeprazole for longer than 1 year, your doctor will regularly check your health to see if you should carry on taking it.

It's not known if omeprazole works less well the longer you take it.

If you feel like omeprazole is not working any more, talk to your doctor.

Omeprazole and stomach cancer

There is some research to suggest that taking medicines to reduce stomach acid, like PPIs and H2 blockers, may slightly increase the chance of developing stomach cancer. It also suggested that it could be more likely in people taking them for longer than 3 years. But studies involving more people need to be done to be sure that PPIs and H2 blockers cause stomach cancer, rather than something else causing it.

PPIs, like most medicines, have side effects so it's best to take them for the shortest time possible.

It’s also important to speak to your doctor if you notice any of these symptoms which can be signs of stomach cancer:

Speak to your doctor or pharmacist if you are concerned.

Other side effects

These are not all the side effects of omeprazole. For a full list, see the leaflet inside your medicine packet.


You can report any suspected side effect using the Yellow Card safety scheme.

Visit Yellow Card for further information.

Page last reviewed: 18 November 2021
Next review due: 18 November 2024