The research cross-referenced stool samples with Sweden’s prescribed drug registry to compare the gut microbiomes of people who had taken antibiotics at some point in the previous eight years with those who hadn’t
On average, people who had not taken any antibiotics in the past eight years had about 350 unique bacterial species living in their gut, but those who had taken any antibiotics in that time frame had fewer. The level of microbial diversity also depended on which drug they had taken.
Clindamycin, which is sometimes prescribed for skin and dental infections, was the most disruptive: each course taken in the year before stool sampling was linked to an average of 47 fewer detected species, as well as changes in abundance in almost 300 of the 1,340 species analyzed.
Courses of fluoroquinolones, which are often prescribed for urinary tract infections and respiratory infections, and flucloxacillin, which is mainly prescribed for Staphylococcus aureus infections, both corresponded to an average of about 20 fewer species. (Flucloxacillin is not available in the U.S.) They were also linked to changes in the abundance of 172 and 203 species, respectively. Most of the other antibiotics analyzed in the study were linked to decreases in bacterial abundance, but some were linked to increases in bacteria that have been associated with poor cardiometabolichealth.
In the study, the antibiotics’ effects were strongest when they were taken in the year immediately before the stool sampling. In general, the more courses of antibiotics people took, the larger the change in microbial diversity was. But even a single course of one of these three medications, taken up to eight years earlier, had an effect.
“It seems like you don’t recover completely,” says Tove Fall, the study’s senior author. That finding is consistent with other smaller and shorter studies.
Clindamycin and fluoroquinolones are “broad-spectrum” antibiotics—they target a broader range of bacteria—and they reach high concentrations in the colon, which might explain their outsize effects, Fall says. But flucloxacillin, a “narrow-spectrum” penicillin, was a surprise, she says; its effect might be related to its variable bioavailability and only partial bile excretion, meaning more of it ends up in the large intestine.
There’s just no evidence that probiotics are the answer
