The researchers found that as people's antibiotic use increased, their odds of being diagnosed with colon cancer inched up. Specifically, the risk was tied to antibiotics that kill anaerobic bacteria -- which include common drugs like penicillins and cephalosporins such as Keflex.
The findings add to recent research hinting at a link between antibiotics and colon cancer.
The reasons for the connection, however, remain unclear -- and it might not reflect a direct effect of antibiotics at all, according to one colon cancer expert.
"We don't know why people in this study received antibiotic prescriptions," said Dr. Emmanouil Pappou, a colon cancer surgeon at Memorial Sloan Kettering Cancer Center, in New York City.
In particular, he noted, the risk was greatest among people who'd used antibiotics for longer periods -- 30 to 60 days, or longer. And those patients might have been quite sick, said Pappou, who was not involved in the study.
"There are many unanswered questions at this point," Pappou said. "More research is needed to confirm whether this is a true effect of antibiotics."
Why would antibiotics affect colon cancer risk? In theory, through effects on the gut microbiome, according to senior researcher Dr. Cynthia Sears.
The gut microbiome refers to the vast collection of bacteria and other microbes that naturally populate the colon. Recent research has been suggesting that the balance of microbes there may have broad effects on health.
And it's possible, according to Sears, that disruptions in the gut microbiome could make way for the growth of bacteria with cancer-promoting potential. Research has suggested that certain bacteria, such as E. coli, might contribute to colon cancer in some cases.
Like Pappou, Sears stressed that her findings do not prove cause and effect. But they make yet another argument for judicious antibiotic use, she said.
"Antibiotic overuse is a problem, and education is needed on both the patient and doctor side," said Sears, a professor of medicine at Johns Hopkins University, in Baltimore.
That means not taking antibiotics for viral infections like the common cold. (Antibiotics target bacteria, not viruses, Sears pointed out.) And long courses of antibiotic treatment should be avoided whenever possible, she added.
The report was published online Aug. 21 in the journal Gut.
The results are based on over 166,000 middle-aged and older primary care patients whose information was collected between 1989 and 2012. Nearly 29,000 were diagnosed with colon or rectal cancer at some point; each of those patients was compared with up to five others who remained free of the cancers.
The researchers looked at any antibiotic prescription patients received from the time they enrolled in the study, up until a year before a colon cancer diagnosis.
On average, the findings showed that patients who'd received a prescription were at slightly higher risk of colon cancer -- 9% higher if they'd been on antibiotics for a total of two weeks, versus no prescription. People who were on the medications for more than 60 days total had a 17% higher risk.
In contrast, antibiotic use was linked to a small decrease in the risk of rectal cancer.
The reasons for that are unclear. But, Sears said, it may reflect the fact that colon and rectal cancers are distinct from one another.
Pappou, however, cautioned against making too much of the results -- partly because antibiotic use was linked to only small increases in colon cancer risk.
But he agreed on the need for wise antibiotic use. "Only use them for a valid medical reason," he said.
When it comes to cutting colon cancer risk, Pappou stressed the established measures: regular exercise, maintaining a healthy weight, not smoking, limiting alcohol, and eating a diet rich in fruits, vegetables and fiber-rich grains.
Sears pointed to another critical step: Getting the recommended colon cancer screenings. If you have a history of antibiotic prescriptions, she noted, that may give you even more incentive.