Yes, it can. The NHS lists constipation as a common side effect of omeprazole, alongside diarrhoea, wind and stomach pain [1]. Several other everyday medicines can slow your bowels too, from codeine-based painkillers to iron tablets, so if things changed when a new prescription started, it is worth talking to your pharmacist.
Omeprazole can cause constipation: the NHS lists it among the common side effects, which affect more than 1 in 100 people [1]. Lansoprazole, co-codamol, alginate antacids such as Gaviscon and iron tablets can also cause it [3][4][5]. Do not stop a prescribed medicine on your own; speak to your GP first [2].
Does omeprazole cause constipation?
Omeprazole is a proton pump inhibitor (PPI), a medicine that reduces the amount of acid your stomach makes, often used for acid reflux and stomach ulcers [1]. Its common side effects, seen in more than 1 in 100 people, include headaches, stomach pain, feeling or being sick, constipation or diarrhoea, and farting more than usual [1]. Not everyone gets them. Unsure whether your pattern has really changed? Start with how often you should poo.
Lansoprazole and pantoprazole belong to the same family. The NHS notes that side effects of PPIs are usually mild but can include diarrhoea or constipation, feeling sick, stomach ache and dizziness [3]. Medicines can shift your gut in other ways too, as we explored with GLP-1 medicines and digestion.
Which other medicines can cause constipation?
The NHS lists a side effect of medicine as one of the common causes of constipation, and suggests seeing a GP if you are taking a medicine that is causing it, such as opioid painkillers [2]. Here is how some everyday medicines compare:
| Medicine | Often used for | What the NHS says |
|---|---|---|
| Omeprazole | Acid reflux, stomach ulcers | Constipation or diarrhoea is a common side effect [1] |
| Lansoprazole | Acid reflux, stomach ulcers | Diarrhoea or constipation listed among PPI side effects [3] |
| Co-codamol | Pain | Constipation is one of the most common side effects [4] |
| Gaviscon and other alginates | Heartburn, indigestion | Usually minor side effects, which can include diarrhoea or constipation [3] |
| Iron tablets | Iron deficiency anaemia | Constipation and darker than usual poo are common [5] |
Iron deserves a special mention. A meta-analysis of randomised trials found ferrous sulfate roughly doubled the odds of gut side effects compared with placebo [7]. Darker poo is expected, but the NHS says to talk to a doctor or call 111 straight away if your poo is black and tar-like, red, or has blood in it [6]. Our guide to black stool explains the difference.
What helps if your medicine is making you constipated?
First, do not stop a prescribed medicine without speaking to your GP [2]. Meanwhile, the NHS suggests gradually increasing fibre, drinking plenty of fluids, adding oats or linseed, and a daily walk [2]. Read more on fibre and gut health. A pharmacist can suggest a laxative if needed; most work within three days and are meant for short-term use [2]. Our guide to natural laxatives covers food-first options.
Medicines and constipation: your questions answered
Does Gaviscon cause constipation?
It can. The NHS describes the side effects of antacids and alginates such as Gaviscon as usually minor, and lists diarrhoea or constipation among them, alongside wind, stomach cramps and feeling sick [3]. If you rely on it most days, it is worth asking your GP or pharmacist whether your indigestion needs a closer look.
Do iron tablets make your poo black?
They often make it darker than usual, which is a recognised side effect [5]. Poo that is black and tar-like, red, or has blood in it is different: the NHS advises talking to a doctor or calling 111 straight away, especially if you also feel unwell [6]. Keep taking your iron unless you are advised to stop.
Can omeprazole make you bloated?
It may feel that way for some people. The NHS lists stomach pain and farting more than usual among omeprazole's common side effects [1]. If bloating appeared after you started it, mention it to your pharmacist or GP. For other causes, see our guide to antibiotics and gut health and how other medicines can affect your gut.
Keep a simple note of when you started a new medicine and any change in how often you go or what your poo looks like on the Bristol Stool Chart, and share it with your pharmacist or GP. Small, kind changes, like a walk after breakfast or a bowl of porridge, may help while you talk it through. Never stop a prescription without advice.
References
- NHS. Omeprazole. nhs.uk/medicines/omeprazole
- NHS. Constipation. Last reviewed 26 October 2023. nhs.uk/conditions/constipation
- NHS. Stomach ulcer: treatment. nhs.uk
- NHS. About co-codamol for adults. nhs.uk
- NHS. Side effects of ferrous fumarate. nhs.uk
- NHS. Side effects of ferrous sulfate. nhs.uk
- Tolkien Z, Stecher L, Mander AP, Pereira DIA, Powell JJ. Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis. PLoS ONE. 2015;10(2):e0117383. doi:10.1371/journal.pone.0117383
This content is for informational purposes only and does not constitute medical advice. If you have concerns about your digestive health, please speak to a qualified healthcare professional.
