One course of antibiotics can reshape your gut for years.
- Adam Spencer
- 14 hours ago
- 3 min read
A Swedish study of 15,000 adults finds antibiotics may reshape the gut microbiome for up to eight years. Miracle drugs, but with a microbial hangover.
An ecosystem inside you.
Your body is home to an entire ecosystem. It’s called your gut microbiome.
You've probably heard Instagram influencers talk about it in connection with everything from getting a good night’s sleep to having world-class bowel movements.
And it is certainly true that we are only just beginning to understand its complexity and how it may affect our daily lives.
Here's one amazing fact we do know about your gut microbiome.
It comprises a lazy 40 trillion microscopic organisms, each just a few microns across.
In case you’re wondering that’s slightly more than the total number of cells in your body.
As my old friend Dr Karl used to say;
“It is quite possible that the first aliens to see us could mistake humans for an incredibly efficient, warm, nutrient‑packed mobile home for the super‑population in our gut.” Dr Karl Kruszelnicki.
Well while we can now map it in extraordinary detail we still can’t control it.
And it turns out, the same drugs we take to save lives can reshape it for years.
The miracle, followed by the mess.
Antibiotics are one of medicine’s greatest inventions.
They have saved hundreds of millions of lives.
But they do not just kill the bacteria causing your infection.
They also bulldoze the microscopic ecosystem living inside your gut.
A new study of almost 15,000 Swedish adults, published in Nature Medicine on 11 March 2026, suggests that impact may last far longer than most doctors assumed.
Researchers matched stool samples with Sweden’s national prescription database, allowing them to track antibiotic exposure over eight years. I know, slightly gross. But really cool.
Because it showed that the difference was clear.
People who had avoided antibiotics hosted roughly 350 bacterial species in their gut on average. Those who had taken them had fewer.
What the flucloxacillin?
Not all antibiotics behaved the same.
Clindamycin, often prescribed for dental or skin infections, was the most disruptive.
A single course taken in the year before sampling corresponded to an average loss of 47 bacterial species, and threw hundreds of others out of balance. Some wiped out, others unexpectedly taking over. Significant gut disruption.
Fluoroquinolones and flucloxacillin were also linked to roughly 20 fewer species, and similar knock-on disruptions across the gut ecosystem.
“We can see that antibiotic use as far back as four to eight years ago is linked to the composition of a person’s gut microbiome today.” Gabriel Baldanzi, Uppsala University.
You’ll bounce back … well, sorta.
The gut does fight back.
Microbial diversity rebounds most quickly in the first two years after antibiotics. But then recovery slows. And it appears incomplete.
“It seems like you don’t recover completely.” Tove Fall, Professor of Molecular Epidemiology Uppsala University.
Previous research has linked reduced microbial diversity to obesity, inflammatory bowel disease and type-2 diabetes. But scientists are still arguing about what actually matters most: overall diversity, or specific species.
What this new dataset shows clearly is persistence.
Antibiotics do not just cause a short shock to the system. They can leave a long ecological footprint.
Cool. I never trusted anti-biotics anyway.
Slow down there cowboy. Before anyone swears off antibiotics, the researchers are blunt.
These drugs remain essential.
Without them, routine infections, surgeries and cancer treatments would become far more dangerous.
The lesson is not to avoid antibiotics when needed. It is to recognise what they are.
Ecological bulldozers.
Sometimes you need a bulldozer. But you do not use one to tidy the garden.
Have a healthy, hopefully anti-biotic free, day.
And say hi to your 40,000,000,000,000 little biome-buddies for me.
Hey I'm now also on substack.
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