Do laxatives affect the gut microbiome?
If you use laxatives to manage constipation, you may have wondered what they do to the trillions of bacteria living in your gut. The short answer is that it depends on the type of laxative, how it is used, and for how long. Understanding the difference can help you use laxatives more confidently.
How do Laxatives affect the gut microbiome?
The gut microbiome — the community of bacteria and other microorganisms that live in the gut — is sensitive to changes in the gut environment, including shifts in temperature, pH, osmolality (the concentration of dissolved particles in gut fluid), water content, and transit speed. 1 Because laxatives work by changing this gut environment — and they do so in different ways — their effects on the microbiome also differ. Some appear disruptive, while others may even support beneficial bacteria.
Osmotic laxatives, such as polyethylene glycol (PEG), work by drawing water into the bowel and increasing stool bulk and speed of transit. 1 In clinical practice, the high-dose polyethylene glycol (PEG) electrolyte solutions used for colonoscopy preparation have been shown to cause a sharp short-term drop in gut microbial load and diversity, with most of the microbiota recovering within about two weeks.2
Lactulose has a distinctly different profile. Because lactulose reaches the colon undigested, it acts as a prebiotic — selectively feeding beneficial bacteria, particularly Bifidobacterium and Lactobacillus. 3 Studies have found that lactulose can rapidly increase Bifidobacterium levels in healthy adults, even at doses lower than those used to treat constipation. 4 Far from harming the gut microbiome, lactulose appears to support a favourable microbial environment.
Do laxatives destroy gut bacteria?
It is sometimes suggested that laxatives could deplete the gut microbiota. The concern often arises from studies of bowel preparation agents — high-volume osmotic solutions used to flush the colon before colonoscopy. These are used at doses exceeding those used for constipation, and although they cause a substantial reduction in gut bacteria, the microbiome typically recovers within about two weeks.2
Osmotic laxatives such as lactulose or PEG used at constipation doses do not flush the colon in the same way as bowel preparation. As described above, lactulose at low doses is associated with a beneficial rather than harmful microbiome effect. 4
Stimulant laxatives — such as bisacodyl and senna — work differently. They stimulate the nerves and muscles of the colon wall, increasing motility and secretion. 5 A recent review concluded that the harms of stimulant laxatives have likely been overstated, finding no strong evidence that they damage the gut itself, even with long-term use. 5
Can stimulant laxatives harm your gut bacteria?
With regard to the gut microbiome specifically, there is little direct research on stimulant laxatives. The wider safety research has looked at the gut lining, nerves, and cancer risk, and does not point to harm when these laxatives are used as directed. 5 Taking much more than the recommended dose has been linked to changes in the gut lining, but these changes appear to be temporary, and using these laxatives long term does not appear to cause lasting harm. 5
One point worth noting is that senna depends on colonic bacteria to work. Sennosides — the active ingredients in senna — pass through the stomach and small intestine unchanged and only become active when bacteria in the colon convert them into rhein anthrone, the compound that produces the laxative effect. 6 This means changes in the microbiome could theoretically alter how senna works, but it does not mean senna itself damages gut bacteria.
What you can do
Direct evidence on how laxatives at the doses used for constipation affect the gut microbiome is limited. What is clearer is that after high-dose bowel preparation (for example, before a colonoscopy), the gut microbiome usually recovers on its own within about two weeks.2 More generally, if you would like to support your gut microbiome — whether or not you use laxatives — the following strategies may help:
- Increase dietary fibre. Dietary fibre acts as a prebiotic — it feeds beneficial bacteria in the colon. One study in adults with functional constipation found that fibre supplements (polydextrose, psyllium husk, or wheat bran combined with psyllium) significantly improved stool consistency and were associated with increases in beneficial bacteria such as Bifidobacterium and Anaerostipes. 7
- Consider probiotics. A review of clinical trials found that probiotic-containing products improved both stool frequency and gut microbiota composition in adults with functional constipation, with increases in Bifidobacterium among the most commonly observed changes. 8 Where probiotics are used, products containing multiple bacterial strains have shown more reliable benefit than single-strain products in clinical trials. 8 Discuss with a healthcare provider whether a specific probiotic formulation is appropriate for you.
Conclusion
If you are using laxatives for constipation, choosing one that is consistent with good gut health makes sense. Lactulose — an osmotic laxative that acts as a prebiotic for beneficial gut bacteria alongside its laxative effect 4 — is an option you may want to discuss with your pharmacist or doctor. Where appropriate, increasing dietary fibre can support stool consistency and gut microbiome composition. 7
FAQ
It depends on the laxative and the dose. In studies of the high-dose laxatives used before colonoscopy, most of the gut microbiome returns to normal within about two weeks.2 If you use laxatives long term and are concerned about your gut health, your pharmacist or doctor can advise you.
There is no clear evidence that probiotics are needed when using laxatives at standard doses. Probiotics can help with constipation symptoms in their own right, particularly products containing multiple bacterial strains 8, but they are not a routine companion to laxative use. If you are considering one, ask your pharmacist or doctor for advice.
Yes. Lactulose stands out because it acts as a prebiotic at low doses — feeding beneficial bacteria rather than depleting them. 4 For other laxatives at standard doses, there is little direct research on microbiome effects, and the broader safety evidence does not suggest harm to the colon at standard doses. 5
For everyday laxatives used at standard doses, the evidence is broadly reassuring. A recent review of stimulant laxatives such as senna and bisacodyl found no convincing evidence of harm to the colon, even with long-term use. 5 Lactulose, an osmotic laxative, has a favourable profile because at low doses it can support the growth of beneficial gut bacteria. 4 If you find yourself relying on laxatives often, your pharmacist or doctor can help you look at the underlying cause.
Yes. Eating more fibre feeds beneficial gut bacteria and can also improve stool consistency. 7 Probiotic products may also help, especially those containing several different bacterial strains. 8 Used together, diet and laxatives can be a sensible way to manage constipation.
This article was written with the assistance of generative AI technology and reviewed for accuracy.
