Steroid tablets such as prednisolone reach the gut in three main ways. They can change the mix of gut bacteria, which a first trial in healthy adults has now measured directly. They can irritate the stomach, with the serious risks concentrated in people who also take anti-inflammatory painkillers such as ibuprofen. And by suppressing the immune system they make some infections more likely, including thrush in the mouth and gullet. Most people on a short course notice little beyond indigestion or a bigger appetite, and nothing on this page is a reason to stop a prescribed steroid, because stopping suddenly is dangerous in itself. This guide covers which steroids reach the gut, what the research shows about gut bacteria, stomach and infection risks, and which symptoms need a GP.
Which steroids affect the gut?
Corticosteroid is the full name for these medicines, which are built to copy cortisol, a hormone your adrenal glands make to regulate inflammation, metabolism and the immune system. They are not the anabolic steroids linked with bodybuilding. In the UK the most commonly prescribed steroid tablet is prednisolone, used for asthma attacks, flare-ups of arthritis and inflammatory conditions, and much else. Dexamethasone and hydrocortisone tablets do similar jobs at different strengths.
How much a steroid touches the gut depends on how it enters the body. A tablet is absorbed and carried everywhere, including to the gut lining and the immune tissue inside it. Inhalers, nasal sprays, creams and joint injections deliver far smaller amounts to the rest of the body, so tablets are where the gut questions mainly arise. One steroid, budesonide, is deliberately released inside the gut and broken down quickly after absorption, and specialists prescribe it precisely because gut inflammation is what it treats.
Steroids also work in the other direction. Flare-ups of Crohn's disease and ulcerative colitis are often brought under control with a steroid course, making steroids a treatment for gut disease as well as a medicine with gut side effects. The NHS guide to steroids covers the forms they come in and why they are prescribed.
Do steroid tablets change your gut bacteria?
Until recently nobody had tested this properly in people. Population studies comparing medicine users with non-users have linked many everyday medicines to differences in gut bacteria. In a Dutch study comparing users and non-users of 41 medicine types, the strongest links belonged to antibiotics, acid reducers, metformin and laxatives, and a UK study of more than 2,700 people reached a similar broad conclusion across 51 medicines. Steroid tablets received far less of that attention, partly because the people who take them are often unwell in ways that change the gut by themselves.
A randomised trial has now looked directly. In a 2025 Danish trial, 56 healthy young men were randomly assigned to a week of prednisolone tablets, a single steroid injection into muscle, or a dummy treatment. After a week of tablets the mix of gut bacteria had measurably shifted, with some groups becoming more common and others declining, and the changes tracked alongside early markers of raised blood sugar and a quietened immune response. The injected steroid barely changed the gut bacteria at all, so the drug passing through the gut itself seems to be what matters. This is the first trial of its kind, it lasted a week, and it studied young healthy men, so it needs repeating before firm conclusions are drawn.
The picture in unwell patients is less tidy. When 27 patients with severe alcohol-related liver inflammation took prednisolone for a week, their gut bacteria did not measurably change at all. People this ill start with a heavily disturbed gut, which may drown out any effect of the medicine. The fair summary is that a steroid course probably nudges gut bacteria rather than devastating them, and that long courses have not been properly studied in people.
Animal research points the same way, with stronger effects. In a study in mice, eight weeks of prednisolone changed the gut bacteria, made the gut lining leakier, and thinned the bones, and the bone loss largely failed to happen when the gut bacteria were removed or the gut lining was protected. That suggests the gut may be part of how steroids weaken bones, which is one of their best-known long-term side effects. Whether the same chain of events runs in people taking prednisolone has not been shown.
Can steroids cause indigestion or stomach ulcers?
Indigestion and heartburn are among the most commonly reported side effects of steroid tablets, which is why the patient leaflet says to take them with or after food. For most people that is as far as it goes.
Whether steroids on their own cause ulcers was argued over for decades. The modern studies point to little risk from a steroid alone and a much larger risk from one combination. In a study of 1,415 people hospitalised with ulcers or stomach bleeding, compared with over 7,000 without, people taking a steroid alone had little or no extra ulcer risk. People taking a steroid together with an anti-inflammatory painkiller had around 15 times the risk of people taking neither. The two medicines each remove a different layer of the stomach's protection, and it is the combination that does the damage. Our guide to what ibuprofen and other NSAIDs do to the stomach and gut covers the other half of that pairing.
The trial evidence agrees on the size of the risk. A review pooling 159 randomised trials in over 33,000 people found stomach or gut bleeding in 2.9% of people given a steroid against 2% given placebo, with the increase concentrated in hospital patients who were seriously ill. In people treated outside hospital, bleeding was rare, affecting roughly one person in 750, and the difference from placebo was not statistically clear.
In practice, a GP weighing up a longer steroid course alongside other risk factors may add a stomach-protecting medicine, usually omeprazole or a similar acid reducer. Those medicines have gut bacteria effects of their own, which we cover in our guide to what proton pump inhibitors such as omeprazole do to the gut. The practical point for anyone on steroid tablets is simpler. Check with a pharmacist before adding any anti-inflammatory painkiller, including ibuprofen bought over the counter, because that combination is where the stomach risk lives.
Why do infections become more likely on steroids?
Damping the immune system is what steroids are for, and the protection you trade away includes protection against infection. An analysis pooling 71 trials found infections in 12.7% of people taking steroid tablets against 8% of those on placebo, with the risk climbing as doses rose and little or no rise at the lowest doses. Sicker patients had more infections whichever treatment they took, so the underlying illness carries part of the blame.
This matters for the gut twice over. The gut holds the largest collection of immune tissue in the body, sitting one cell layer away from trillions of bacteria, and steroids quieten it along with the rest of the immune system. And when infections do arrive, they are often treated with antibiotics, which have well-mapped effects on gut bacteria of their own. We cover what a course of antibiotics does to gut bacteria and how to help your gut recover afterwards in separate guides. Someone on long-term steroids who picks up repeated infections can end up with their gut bacteria shaped as much by the antibiotic courses as by the steroid itself.
Can steroids cause thrush in the mouth, throat or gut?
Yes, and it is one of the clearest gut-related effects. Candida is a yeast that lives harmlessly in most people's mouths and digestive tracts, kept in check by the immune system and by competing bacteria. Steroids loosen that check. Oral thrush, white patches and soreness in the mouth, is a familiar side effect of steroid inhalers, which is why asthma nurses advise rinsing the mouth with water after each use.
Further down, the same yeast can overgrow in the oesophagus, the tube between mouth and stomach. In a Japanese study of over 80,000 people who had a camera examination of the stomach, people taking steroids had around six times the odds of candida overgrowth in the oesophagus, and the risk rose with higher steroid doses. It usually shows up as pain or discomfort when swallowing, it is treatable, and it needs a clinician to diagnose it. If swallowing becomes painful while you are taking a steroid, that is a GP appointment, not something to watch and wait on.
What can a gut microbiome test show if you take steroids?
A stool test cannot diagnose thrush, an ulcer or any other condition, cannot measure how suppressed your immune system is, and cannot tell you whether a steroid caused whatever it finds. If you have symptoms, a test is not a substitute for seeing a GP.
What it can show is the current state of your gut bacteria, including overall variety and the levels of the fibre-fermenting groups the research above keeps returning to. If steroids have been part of your life for months or years, perhaps with antibiotic courses along the way, a gut microbiome test measures the variety and levels of your gut bacteria from one home-collected sample, and gives you something specific to discuss with your GP or pharmacist. Our page covering what a gut microbiome test can tell you about a gut imbalance explains what the results include and what they leave out.
How can you look after your gut while taking steroids?
None of the following means changing how you take a prescribed steroid.
- Take the tablets exactly as prescribed, with or after food. If you have been on steroids for more than a few weeks, never stop suddenly, because your body pauses its own cortisol production and needs time to restart it. Doses are reduced gradually, on the prescriber's plan
- If you take long-term steroids, carry the NHS steroid emergency card and mention the medicine to every clinician, dentist and pharmacist you see
- Ask a pharmacist before adding any painkiller. Ibuprofen and aspirin from the supermarket count, and the steroid combination is the one worth avoiding without advice
- Rinse your mouth with water after a steroid inhaler, and report white patches or a sore mouth rather than putting up with them
- Feed your gut bacteria. The fibre-fermenting groups run on plant variety, and our guide to which foods feed your gut bacteria best sets out where to start
- Expect your appetite to rise on longer courses. Regular meals built around plants, protein and fibre serve your gut bacteria and the extra hunger better than grazing does
- If you are pregnant or breastfeeding, check any steroid question with your midwife or GP, including one you have taken before
When should you see a GP?
Two situations need urgent help rather than an appointment. Vomiting blood, vomit that looks like ground coffee, or black tarry stools can mean stomach bleeding, and the risk is highest if you also take anti-inflammatory painkillers. Call 999 or go to A&E. The second is a cortisol emergency. If you have been on steroids for weeks and become very unwell with vomiting, weakness or collapse, particularly after missed doses or a sudden stop, seek immediate help and show your steroid card.
See a GP promptly, whatever dose you take, if any of the following apply.
- A high temperature or other signs of infection, because steroids can both raise the risk and soften the warning signs
- Pain or difficulty swallowing, or white patches in the mouth, which can mean thrush needing treatment
- New or worsening stomach pain, or indigestion that does not settle with food and the pharmacist's advice
- Chickenpox, shingles or measles contact while on steroid tablets, if you have not had chickenpox before, because these infections can be severe when the immune system is damped
- Blood in your stool, unexplained weight loss, or a bowel habit that has changed and stayed changed for over three weeks, especially past 50 or with a close family history of bowel or ovarian cancer. These are gut red flags in anyone, on steroids or not
Steroid treatment in children is planned and monitored by a GP or specialist, and doses should never be adjusted at home. The same chickenpox and measles caution applies with more force, so contact the GP promptly about any contact while a child is taking steroid tablets.
This guide is for information only and does not replace medical advice. Speak to your GP or pharmacist about your own symptoms and treatment.
Sources
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