Medication and the gut: when the cause sits in the pill box

You have a list in your head that you have been working on for months. Wheat gone. Milk gone. Onions gone. A stool test, a breath test, an ultrasound. In between a few weeks when it was better, and then not again.

On the bedside table a packet has been lying there the whole time. Maybe a painkiller for your back. Maybe a diabetes medicine you have been taking since the spring. Maybe something for heartburn that was once meant for four weeks and is now two years old.

Many people know exactly this pattern. The gaze goes inwards, deeper, finer. It rarely goes to the bedside table.

That is no reproach to anyone. Medicines are prescribed because they are needed, and in the vast majority of cases rightly so. But they almost never sit on a sheet of paper with a date, and without a date no temporal connection can be seen.

The look that is almost always skipped

Why is this look so often left out? Not out of carelessness. But because the medication list is spread across several heads.

The back belongs to orthopaedics. The blood sugar to diabetology. The mood to psychiatry. The belly to gastroenterology. Every practice knows its part and rarely sees the sum. On top of that come the preparations that do not count as a medicine at all: the magnesium from the chemist, the iron from the internet, the antacid in the handbag.

Precisely for that reason the complete list is not an act of bureaucracy. It is the first diagnostic step that you can prepare yourself.

The German S3 guideline on irritable bowel syndrome, carried jointly by the DGVS and the DGNM, lists the medication history as a standing component of the workup for good reason Guideline [Guideline]. That is no side note. It is the acknowledgement that a considerable share of gut symptoms has an explainable external cause.

And the second half of that sentence matters just as much. If the list shows something, no stopping follows from it. A conversation follows.

The difference is not formal. A medicine has a reason. A blood pressure drug keeps a blood pressure down, an antidepressant keeps a depression in check, a painkiller makes movement possible again. Anyone who deletes that on their own trades one problem for another, and the second is often bigger.

One more word on attitude. This article is no charge against medicines. It is also no call to take fewer of them. Non steroidal anti inflammatory drugs make movement possible where otherwise nothing works. Metformin is among the best studied substances in internal medicine. Opioids are often without an alternative in severe pain. Antidepressants bring people out of states that otherwise do not end.

What is additionally true: they all have side effects, and part of that plays out in the digestive tract. Holding both thoughts at once is no contradiction. It is medicine.

And now you know why the list belongs before the next test.

What the big screening found, and what it does not say

There is a number that haunts half the internet. A quarter of all medicines damage the gut flora. The number is not invented. It is just often told wrongly.

So the honest question is: is there a counter check in humans? Yes, there are two.

And now you know why the famous number from the screening is a beginning and not a diagnosis.

NSAIDs: the stomach is only half the story

That ibuprofen and diclofenac can be hard on the stomach is well known. Less well known is that, according to the data from capsule endoscopy studies, the small intestine can be affected at least as often.

First the most important thing, so that the order is right. Non steroidal anti inflammatory drugs are one of the most useful drug groups we have. They turn a shoulder immobilised by pain into a moving one. They keep people with inflammatory rheumatic diseases going in daily life. Low dose aspirin can lower the risk of a further event after a heart attack or a stroke, and large pooled analyses support that well. None of that is up for debate here.

Because this chapter concentrates on the mucosa, the second problem area should at least be named. Painkillers of the NSAID type can also put a strain on the kidney, raise blood pressure and hold water in the body. The combination of an NSAID, a blood pressure drug of the ACE inhibitor or sartan type and a water tablet is considered particularly risky. If you take these three groups together, that belongs on the list and in the conversation, and with longer use the kidney values belong under regular control.

That these changes are not theory has only been known since it became possible to look through the small intestine with a swallowable camera. Before that, every endoscopy ended in the duodenum and started again in the large bowel. In between lay about five metres that nobody saw.

These 71 percent do not stand alone. They stand in a row, and the row is instructive because the numbers diverge.

Gastric protection protects at the top. Not further down.

Now comes the part that rarely turns up in advice articles. It answers the search for a stomach friendly painkiller, and the answer is uncomfortable.

And now the sentence that matters just as much as the finding. This is a cross sectional analysis. It shows a connection, not proof of a cause. People who are given an acid blocker often carry a higher risk from the outset, otherwise nobody would have prescribed it.

In practice this means: if you take NSAIDs regularly and have abdominal symptoms, anaemia or unclear iron losses alongside, that is a constellation for a conversation. There are medical options, from checking the indication through the choice of substance to further diagnostics. Which of them fits is decided by the prescribing practice. And anyone who needs NSAIDs because of an illness does not stop them on their own.

And now you know why the question about the painkiller in gut symptoms must not stay limited to the stomach.

Acid blockers and antibiotics: brief, because they have their own articles

Two groups appear at the top of every list. Both have their own article in the ViveCura cluster, so here it stays with what the system needs.

Acid blockers. The stomach is the first barrier against everything you swallow, and less acid means that more oral bacteria arrive alive further down. In an analysis of 1,815 people from three cohorts, diversity in the stool fell under proton pump inhibitors, and oral genera such as Rothia increased clearly, which the authors place as consistent with the known higher risk of gut infections Human [Cohort, human]. Everything further, from the indication through tapering to rebound, is described in heartburn and understanding acid blockers. And if the question of too little rather than too much acid interests you: low stomach acid and betaine HCl.

Imhann F, Bonder MJ, Vich Vila A et al. Gut. 2016;65(5):740-748. PMID: 26657899 · DOI: 10.1136/gutjnl-2015-310376 [Cohort, human]

Antibiotics. Twelve healthy men received a triple combination of reserve antibiotics for four days, after which their microbiome was followed for six months. After about 1.5 months the composition was close to the starting point again, but nine species that had been present in all twelve beforehand were still missing in most after 180 days. So the big picture returns within weeks, individual residents do not, and with twelve young healthy men and high dose reserve antibiotics the study is expressly not transferable to a week of amoxicillin Human [Cohort, intervention study, human, small]. What a sensible rebuild afterwards can look like is described in the gut after antibiotics.

Palleja A, Mikkelsen KH, Forslund SK et al. Nat Microbiol. 2018;3(11):1255-1265. PMID: 30349083 · DOI: 10.1038/s41564-018-0257-9 [Cohort, intervention study, human, n=12]

The sentence from above applies here too, and to both groups. An acid blocker has a benefit that is not argued away: it protects the upper digestive tract in reflux disease, in ulcers and under demanding concomitant medication. It is therefore not stopped on your own because of a microbiome finding, it is discussed. And an antibiotic course started for a good reason is not ended early because someone read something about the gut flora on the internet. An incompletely treated infection is the bigger problem. In both cases stopping, tapering or switching is decided by the prescribing practice.

And now you know why these two groups stand here only as signposts.

Metformin: the gut is not a side stage, it is a site of action

There is a sentence I keep reading in diabetes forums. I have got used to everything, only not to this.

What is meant is the diarrhoea. It often comes in the first weeks, often after a dose increase, and in some people it simply does not go away on its own. Along with flatulence, a metallic taste, sometimes nausea.

Again the benefit first. Metformin has been the first line medicine in type 2 diabetes for decades, with one of the broadest data foundations in all of internal medicine. The point here is not whether it should be taken. The point is why it is so often noticeable in the belly.

What has been examined in studies, and who decides about it

The most common question is: is there a form that is better tolerated? There are data on it, and they are thinner than the tone of some guides suggests.

Further adjusting screws that are customary in diabetology and are likewise decided by the prescribing practice: taking the tablet with a meal instead of before it, a slower dose build up and spreading the dose across the day. You change none of that yourself, and none of it appears here with a number, because that is a prescription and not guide content.

And now you know why metformin diarrhoea deserves a question of its own in the consultation.

Opioids: a condition of its own, not a footnote

There is an expectation that is almost always disappointed. The body will get used to it.

For the pain effect that is often true. For the constipation it is not.

And here too the benefit belongs first. In severe pain, after operations, in cancer treatment and in palliative care, opioids are often without a real alternative. People who can sleep and move because of them should keep them.

That a treatment logic of its own follows from this is no fringe opinion. It is written in a guideline.

And now you know why this topic has earned a name of its own and a plan of its own.

Antidepressants and anticholinergic substances: the brake that works both ways

Two people take an antidepressant. One gets constipation, the other diarrhoea. Both are right. They are simply different substances.

The reason lies in two opposing systems in the gut wall. Acetylcholine is the drive of the gut muscle. Serotonin is the timekeeper that speeds the movement up. Whoever blocks acetylcholine slows things down. Whoever raises serotonin steps on the gas.

The twist: the same brake is the treatment elsewhere

And now comes the change of perspective that carries this section. Exactly the effect that causes an unwanted constipation in one person is the treatment principle in another.

Two guidelines place this. The American irritable bowel guideline of the American College of Gastroenterology from 2021 lists gut brain neuromodulators, which include the tricyclics, as a treatment option for global irritable bowel symptoms Guideline [Guideline]. The German S3 guideline on irritable bowel syndrome from the DGVS and the DGNM likewise treats neuromodulators as an option and lists the medication history as a standing component of the workup Guideline [Guideline]. What else belongs to the mechanics of irritable bowel syndrome is described in IBS: finding the causes, and a well studied non drug option is described in gut directed hypnotherapy for IBS.

The safety side that belongs with it

And now you know why two people on the same medicine can have opposite experiences without either of them exaggerating.

The small overlooked ones: iron, magnesium, statins, colchicine

The big groups are now in place. But the list you bring to the appointment rarely consists only of prescription substances. It also consists of what lies in the kitchen drawer.

Iron preparations

The benefit is undisputed. An iron deficiency can make you tired and weak, can impair stamina and concentration and can affect the immune defence. Clarifying it and treating it is right.

One detail that is not printed in bold in any package leaflet and that I consider practically important: iron colours the stool black. That is harmless in itself. But it can mask true tarry stool from an upper bleed. If the stool is shiny black and sticky, if weakness, pallor, palpitations or abdominal pain come with it, that belongs in immediate medical assessment and is not attributed to the preparation.

And a second thought that belongs to the subject of this article: not every iron deficiency is a question of intake, and not every symptom under iron comes from the preparation. A so far unrecognised coeliac disease can explain both at once, the stubborn iron deficiency and the gut symptoms. With that comes the most important practical note in this field: anyone who eats gluten free on their own before the workup can make the diagnosis impossible, because antibodies and tissue findings can regress on a gluten free diet. So the order is diagnostics first, then a change of diet. How this workup proceeds is described in recognising coeliac disease.

And the sentence of this article applies here too: an iron preparation prescribed for a proven deficiency is not stopped, halved or swapped for something else on your own. What can change in preparation, dose, intake rhythm or route is decided by the prescribing practice.

More on tolerability, absorption and alternatives is described in iron tablets and their side effects, iron deficiency despite iron tablets and iron infusion against iron tablets. Why a gut problem can itself disturb iron absorption is described in iron deficiency from an absorption problem in the gut.

Magnesium and aluminium containing preparations

Magnesium salts draw water into the gut osmotically. That is why they are a classic laxative at higher doses, and why a preparation you take on your own initiative can make the stool softer. How strongly depends on the salt form and on the amount, because poorly soluble forms stay longer in the gut lumen. Aluminium containing antacids go the other way and can slow things down. Both groups have their benefit: magnesium in proven deficiency and in defined uses, antacids for rapid acid binding. So the same applies here: what a doctor prescribed you do not change yourself, and what you bought yourself still belongs on the list and in the conversation. The comparison of forms is described in which magnesium is the best.

Statins

Here too the benefit comes first. In people at raised cardiovascular risk statins can lower the risk of heart attack and stroke, and large pooled analyses support that well. That is the reason they are prescribed. Nothing in the following section questions that.

Acarbose

Here the data are thin, and that is why no number appears here. The mechanism, by contrast, can be described clearly: acarbose slows the breakdown of complex carbohydrates in the small intestine so that blood sugar rises more slowly after a meal. What is not broken down travels further back and is fermented there by bacteria. Flatulence, rumbling and soft stool can arise from that. It is the same process you know from the FODMAP context, only triggered by a medicine. That flatter rise in blood sugar after a meal is exactly the benefit. So with acarbose too: symptoms belong in the conversation, the preparation is not left out or changed in dose on your own. How this fermentation works is described in using the FODMAP diet properly and in fibre myths [Mechanism Review, mechanistically plausible, no robust meta-analysis on frequencies verified].

Colchicine

Again the benefit first. Colchicine is an old anti-inflammatory substance with a firm place in the acute gout attack, in familial Mediterranean fever and in pericarditis. It is not prescribed without reason.

Levothyroxine: not a gut problem, but an absorption problem

Levothyroxine replaces a hormone the thyroid no longer makes in sufficient amounts, and an untreated or underdosed underfunction has consequences of its own, from fatigue through weight to the heart. So the benefit is not up for debate. Levothyroxine itself does not cause gut symptoms. It suffers from them. Its absorption can be disturbed by calcium, iron, acid blockers and other preparations, and with several at once these effects add up.

If you take thyroid hormone and still have symptoms despite good values, it is worth looking at both, at the setting and at the intake situation. You do not change the dose yourself and you do not skip a dose to try something out: dose, formulation and intake intervals are decided by the prescribing practice on the basis of the values. More on that in levothyroxine and persisting symptoms. How an acid blocker can also influence iron absorption is described in acid blockers and iron absorption.

A word on microscopic colitis

There is a form of gut inflammation that can only be seen under the microscope and that causes watery diarrhoea although the colonoscopy looks unremarkable. In a Dutch case control study from a general practice database the odds for such a diagnosis were raised under proton pump inhibitors and NSAIDs, compared with colonoscopy negative controls with adjusted values of 10.6 for proton pump inhibitors and 5.6 for NSAIDs, though with very wide confidence intervals up to 64.2, and the authors place part of the remaining associations rather as an amplification of existing symptoms than as a cause Human [Case Control, wide confidence intervals]. An article of its own on this follows in this cluster. Masclee GMC, Coloma PM, Kuipers EJ, Sturkenboom MCJM. Am J Gastroenterol. 2015;110(5):749-759. PMID: 25916221 · DOI: 10.1038/ajg.2015.119.

And now you know why the over the counter preparations belong on the list as well.

The medication timeline: what you bring to the appointment

A consultation rarely lasts long. In that time someone is supposed to understand what has been happening for months. That almost never works by telling it, and it often works on a sheet of paper.

What I describe here is an approach from practice, not a validated instrument. For this specific format there is no study, and I name that expressly [Clinical practice, no study evidence for this format].

What does a doctor do with such a list? Four things, in this order. First, check the temporal fit, that is whether symptom onset and a medication change fit together at all. Second, check the indication, that is whether the medicine is still needed in this form. Third, consider alternatives within the same drug group or sort out intake intervals and timings. And fourth, when it is medically defensible, plan a controlled stop and restart.

To close, once more what stands above everything. If red flags appear, that is blood in the stool, black tarry stool, vomiting blood, unintended weight loss, fever, night time symptoms or a new persistent change in bowel habit from around the age of 45 to 50, then this is not a case for a list but for a prompt appointment. And a recommended colonoscopy, gastroscopy or laboratory workup is not replaced or postponed by anything in this article.

And now you know why a sheet of paper sometimes clarifies more than the next test.

Common questions about medication and the gut

Where this topic connects to the rest of the body

A medicine rarely acts at one place only. It touches absorption, energy, blood formation and what you eat afterwards. These neighbouring topics belong with it.