It is widely assumed that the gut microbiome "bounces back" within weeks of an antibiotic course. That is, at least, what most patients hear from their doctors. The picture painted by a study published in Nature Medicine in March 2026 is considerably different. In a cohort of nearly 15,000 Swedish adults, the authors show that certain antibiotics leave a measurable imprint on gut microbial diversity for at least eight years.
The word "scar" is not hyperbole. It accurately describes what the data reveal: a persistent alteration of the intestinal ecosystem, detectable years after a single exposure.
The scale of the cohort and the methodology
The study analyzed the fecal microbiome of 14,979 participants from the Swedish CArdioPulmonary bioImage Study (SCAPIS), cross-referenced with national prescription registries. This linkage allows each individual's microbial profile to be mapped against a precise eight-year antibiotic history (study).
Three methodological elements deserve emphasis. First, the cohort size: with nearly 15,000 subjects, this is one of the largest studies to link the microbiome to individual antibiotic history. Second, the temporal depth: eight years of prescription data, allowing the distinction between immediate and residual effects. Third, the granularity: analyses are stratified by antibiotic class, number of courses, and time window of exposure.
70% of the study population had received at least one antibiotic in the eight years preceding sample collection. That figure alone conveys the scale of population-level exposure.
Not all antibiotics are created equal
This is arguably the most actionable finding of the study. The impact on microbial diversity varies dramatically from one antibiotic to another.
Average number of fewer microbial species detected in subjects exposed to clindamycin compared to unexposed subjects.
Clindamycin tops the collateral damage ranking. Subjects who had received this antibiotic had an average of 47 fewer microbial species detected compared to unexposed controls. Flucloxacillin and fluoroquinolones (a family that includes ciprofloxacin and levofloxacin) followed with substantial reductions, accompanied by a detectable increase in bacteria associated with inflammation and cardiometabolic risk.
At the other end of the spectrum, penicillin V and nitrofurantoin showed a markedly lower long-term impact. This disparity has direct clinical implications. When two antibiotics are equivalent in therapeutic efficacy, the intestinal damage profile should weigh in the prescribing decision.
Recovery dynamics: fast then asymptotic
The study does not merely measure the initial impact. It maps the recovery trajectory over eight years, and the profile it reveals is instructive.
Microbial diversity rebounds most rapidly in the first two years following exposure. This is the phase during which the intestinal ecosystem shows its greatest resilience: the ecological niches vacated by the antibiotic are progressively recolonized by commensal species.
But after this initial window, the recovery rate slows considerably. Between four and eight years, the "microbial scar" remains statistically detectable in subjects exposed to high-impact antibiotics. The use of high-impact antibiotics four to eight years before sampling was associated with altered abundance in 10 to 15% of all species studied.
This recovery profile resembles an asymptotic curve: rapid initial progress, gradual plateau, with no guarantee of a complete return to baseline. It challenges the implicit assumption that the microbiome possesses unlimited resilience.
What the study does not say
Every observational study has its blind spots, and this one is no exception.
The prescriptions analyzed come from Swedish national registries. They do not capture antibiotics taken abroad, those administered in hospital settings, or self-medication. The actual exposure of some participants may therefore be underestimated.
The question of reverse causation is also relevant. The authors cannot entirely separate the effect of the antibiotic from that of the infection that prompted the prescription. A severe infection can, on its own, disrupt the microbiome. The antibiotic and the infection likely act in concert, but their relative contributions remain difficult to isolate in an observational design.
Finally, the cohort is exclusively Swedish. Prescribing habits, bacterial resistance patterns, and dietary habits vary considerably across countries. The direct transferability of these findings to other geographic contexts warrants caution.
What these data concretely change
The study does not say that antibiotics should stop being prescribed. That would be an absurd reading of data that, incidentally, only concern antibiotics prescribed for justified infections.
What it does say is that the biological cost of an antibiotic prescription extends far beyond the treatment window. And that this cost is not the same for all antibiotics. Clindamycin leaves a measurable imprint eight years later. Penicillin V, much less so.
This finding directly feeds into the concept of antibiotic stewardship: reasoned prescribing that goes beyond efficacy against the target pathogen and incorporates collateral damage to the intestinal ecosystem. The microbiome is not an indifferent bystander to treatment. It is a functional organ whose diversity underpins metabolic, immune, and cardiovascular health.
The open question — and perhaps the most important one — concerns recovery. If eight years are not enough to erase the scar left by certain antibiotics, what interventions could accelerate recolonization? Diets rich in diverse fiber, fecal microbiota transplantation, defined bacterial consortia: these avenues are the subject of active research. None has yet produced definitive evidence in the specific context of long-term post-antibiotic recovery.
One particular case deserves mention. Saccharomyces boulardii, a probiotic yeast, has robust data supporting its use in preventing antibiotic-associated diarrhea and Clostridioides difficile infection (PubMed). Its value lies in its nature: as a yeast, it is unaffected by antibiotics that target bacteria. It can therefore exert its protective effect on the intestinal mucosa during treatment, precisely when conventional bacterial probiotics are themselves destroyed. Common clinical practice involves administering it from the start of antibiotic therapy and continuing for 15 days after the course ends. It is one of the rare cases where probiotic co-administration with antibiotics rests on a solid mechanistic rationale and converging meta-analyses.
Long-term recovery of microbial diversity, however, remains an open challenge. It will likely be one of the major endeavors of clinical microbiology in the coming decade.
Frequently asked questions
References
- Baldanzi G. et al. Long-term impact of antibiotic use on gut microbiome diversity and composition in the Swedish CArdioPulmonary bioImage Study (SCAPIS). Nature Medicine. 2026 (study).
- Jakobsson H.E. et al. Short-term antibiotic treatment has differing long-term impacts on the human throat and gut microbiome. PLoS ONE. 2010;5(3):e9836 (PubMed).
- Palleja A. et al. Recovery of gut microbiota of healthy adults following antibiotic exposure. Nature Microbiology. 2018;3(11):1255-1265 (PubMed).
- Dethlefsen L. and Relman D.A. Incomplete recovery and individualized responses of the human distal gut microbiota to repeated antibiotic perturbation. PNAS. 2011;108 Suppl 1:4554-4561 (PubMed).
- Suez J. et al. Post-antibiotic gut mucosal microbiome reconstitution is impaired by probiotics and improved by autologous FMT. Cell. 2018;174(6):1406-1423 (PubMed).
- Szajewska H. and Kołodziej M. Systematic review with meta-analysis: Saccharomyces boulardii in the prevention of antibiotic-associated diarrhoea. Alimentary Pharmacology & Therapeutics. 2015;42(7):793-801 (PubMed).
