Gut microbiome and drug response !

The bacteria in the gut can switch drugs on, switch them off or make them more toxic, which helps explain why people respond differently.

Updated 2026-11-01

The gut holds a huge community of microbes with their own enzymes. Oral drugs, and drugs that return to the gut in bile, meet these enzymes before or after absorption. Bacteria can activate prodrugs, inactivate drugs, undo the liver's conjugation and change host metabolism indirectly. Antibiotics, diet and disease change the microbiome, and therefore drug response.

How microbes change drugs

Reduction (for example, of azo bonds or of the lactone ring of digoxin)
Hydrolysis and deconjugation (bacterial beta-glucuronidases)
Decarboxylation: a bacterial tyrosine decarboxylase (in Enterococcus faecalis) can turn levodopa into dopamine in the gut, before it reaches the brain (early evidence)
Indirect effects: microbial products such as bile acids act on host receptors that control drug-metabolising enzymes

Digoxin and Eggerthella lenta

Some strains of E. lenta reduce digoxin to dihydrodigoxin, which is inactive. A 2021 review in Clinical Pharmacokinetics links this to a cardiac glycoside reductase gene cluster that is switched on by digoxin in those strains only. Since not everyone carries these strains, it is a model of how both human and microbial genes shape a drug's level.

Enterohepatic recycling and toxicity

The liver attaches glucuronide to many drugs and sends them into bile. In the intestine, bacterial enzymes can remove that group, so the drug is reabsorbed (recycling) or the active form harms the gut lining. The same review describes this for the cancer drug irinotecan (its inactive glucuronide is turned back into the active metabolite SN-38 in the gut, which may add to diarrhoea) and for NSAIDs (mostly animal data), and the influence of the microbiome on immune checkpoint inhibitors.

Why it matters in practice

Starting or stopping antibiotics, a change in diet, bowel surgery or diarrhoea can change the effect of drugs that depend on gut bacteria. The field is still young: for most drugs there is no routine test, but the mechanism is a reason to look again at levels and effect after big changes in gut flora.

Test yourself

Think of the answer, then tap the question to see it.

Which part of sulfasalazine is released by gut bacteria and acts on the colon?

5-aminosalicylic acid; bacteria split sulfasalazine into sulfapyridine and 5-aminosalicylic acid.

Why can an antibiotic raise digoxin levels in some patients?

In patients whose gut bacteria inactivate digoxin, an antibiotic can reduce those bacteria, so more digoxin is absorbed intact.

What do bacterial beta-glucuronidases do to drug conjugates in the gut?

They remove the glucuronide that the liver attached, freeing the drug so it can be reabsorbed or damage the intestinal lining.

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Example drugs

← All of pharmacology

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: Contribution of the Gut Microbiome to Drug Disposition, Pharmacokinetic and Pharmacodynamic Variability (Clinical Pharmacokinetics (open access, PubMed Central), 2021); DailyMed - DIGOXIN tablet (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - SULFASALAZINE tablet (DailyMed (U.S. National Library of Medicine), 2026).

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Representative model, for educational purposes only.