Smoking and the Gut Microbiome: What Cigarettes Do to Your Digestion
Quick answer: Smokers' gut bacteria differ from non-smokers'. In a small study of people quitting, the mix shifted within weeks — more Firmicutes and Actinobacteria, fewer Bacteroidetes and Proteobacteria — and became more diverse, a change that resembles the gut pattern linked with weight gain. A large survey found former smokers' gut bacteria looked like never-smokers'. Smoking also raises the risk of Crohn's disease. The research is young, so treat the details as early findings.
The gut microbiome — the trillions of microbes living in your digestive tract — is linked with immune function, metabolism and disease risk. Smoking changes it, though researchers are still working out how much that matters for health.
How Smoking Reaches the Gut
Researchers suggest several routes:
Swallowed smoke compounds: Some cigarette smoke chemicals dissolve in saliva and are swallowed.
The bloodstream: Nicotine and other absorbed compounds reach the gut lining through the blood.
Gut movement: Nicotine acts on nicotinic receptors in the gut's own nervous system, which can change how fast food moves through — one reason constipation is common for a while after quitting.
Inflammation: Smoking affects the immune cells in the gut lining, which help decide which bacteria thrive.
What Changes in Smokers' Gut Microbiomes
Smokers differ from never-smokers. In a population study of more than 750 Korean men, current smokers' gut bacteria were significantly different from never-smokers', while former smokers' were not.
Quitting shifts the mix. In a small Swiss study that followed 10 people through a quit attempt for 9 weeks, stopping smoking brought an increase in Firmicutes and Actinobacteria, a fall in Bacteroidetes and Proteobacteria, and greater diversity. The authors noted the shift resembled differences seen between obese and lean people, and the quitters gained weight.
We avoid labelling whole bacterial groups as "good" or "bad": each contains many species, and the same group can be linked with health in one setting and disease in another.
The Smoking–Crohn's Disease Connection
Smoking has opposite links with the two main inflammatory bowel diseases. In a meta-analysis, current smokers had higher odds of Crohn's disease (OR 1.76), while current smoking went with lower odds of ulcerative colitis (OR 0.58) and former smoking with higher odds of it (OR 1.79).
Whether the gut microbiome helps explain these links is being studied but not settled. If you have Crohn's disease, quitting is recommended. If you have ulcerative colitis, quitting is still recommended for your overall health — talk to your gastroenterologist so they can watch for flare-ups.
Nicotine's Specific Role
Nicotine acts on nicotinic receptors in the gut's nerves and on immune cells. One idea is that nicotine's effect on a nerve-driven anti-inflammatory pathway could explain part of the ulcerative colitis link, but that is a hypothesis. Nicotine replacement is not a treatment for gut disease. Smoke contains thousands of other chemicals, so changes seen in smokers can't be put down to nicotine alone.
What Happens After Quitting
In the small quitting study, the shift in gut bacteria showed up within the 9-week study period. The larger survey found former smokers' gut bacteria similar to never-smokers', which suggests the change is not permanent — though a snapshot survey can't show how long recovery takes.
Weight gain is common after quitting: in a meta-analysis of 62 studies, quitters gained an average of 4–5 kg after 12 months, with wide variation — some people lost weight and some gained more than 10 kg. Most of the gain came in the first three months. The main drivers are appetite coming back and nicotine's effect on metabolism ending; the gut shift may play a part, but that has not been measured. Our guide to managing weight after quitting has practical steps.
References
- Biedermann L et al. "Smoking cessation induces profound changes in the composition of the intestinal microbiota in humans." PLoS One, 2013. PubMed
- Lee SH et al. "Association between cigarette smoking status and composition of gut microbiota: population-based cross-sectional study." J Clin Med, 2018. PubMed
- Mahid SS et al. "Smoking and inflammatory bowel disease: a meta-analysis." Mayo Clin Proc, 2006. PubMed
- Aubin HJ et al. "Weight gain in smokers after quitting cigarettes: meta-analysis." BMJ, 2012. PubMed
Frequently Asked Questions
Does smoking cause gut problems?
Smoking raises the risk of Crohn's disease and makes it harder to treat, and it changes the mix of gut bacteria. It is also linked with acid reflux and stomach ulcers. If you have ongoing gut symptoms, especially blood in your stool or unexplained weight loss, see your GP.
Why do some people gain weight after quitting?
Mostly because appetite returns and nicotine no longer speeds up metabolism. Quitters gain 4–5 kg on average after a year, with wide variation. Changes in gut bacteria after quitting resemble those linked with weight gain, but how much they contribute isn't known. The health benefits of quitting far outweigh the risks of the weight.
Is the gut microbiome change after quitting permanent?
It seems not. A large survey found former smokers' gut bacteria similar to never-smokers', and a small study saw the mix start shifting within weeks of quitting. How long full recovery takes hasn't been measured.
Does nicotine itself affect gut health?
Nicotine changes gut movement and acts on nerve and immune cells in the gut, and it may explain part of smoking's odd link with ulcerative colitis. That doesn't make nicotine a treatment. NRT avoids the other chemicals in smoke; if you have a gut condition, ask your doctor about the best way to quit.