The largest epidemiological study showed that taking antibiotics may increase the risk of colon cancer 5 to 10 years after taking these drugs.
Previous studies have suggested that antibiotics can cause permanent changes to the gut microbiome, the community of microbes that live in the GI tract, and that these changes may be linked to an increased risk of colon cancer.
The study sample consisted of 40,000 cancer cases. The researchers reported that the increased risk may be specific to cancers in the so-called proximal colon, the part of the colon that connects to the small intestine and begins in the lower right part of the abdomen.
"It's very clear, when we look at the data, that it's very confined to the proximal or right colon," said study lead author Sophia Harled, a cancer researcher at Umea University in Sweden.
In fact, the risk of antibiotic-related cancer was greater in the beginning of the proximal colon, called the "ascending colon," which runs from the lower abdomen to the upper right.
Taking antibiotics for more than six months
People who took antibiotics for more than six months were more likely to develop cancer; According to the study published last Wednesday in the Journal of the National Cancer Institute.
Compared to people who did not take antibiotics; These individuals have a 17% greater chance of developing cancer in the ascending colon.
short-term antibiotic doses
The team found that short-term doses of antibiotics carry a risk of cancer, albeit much lower than what was seen in regimens that lasted for months.
These data may provide another reason to curb overprescribing of antibiotics, Harled said, as well as prevent the emergence of antibiotic-resistant superbugs.
Data from the Swedish Register of Prescription Drugs allowed the team to track these patients' use of antibiotics between 2005 and 2016. They also compared cancer patients to more than 200,000 cancer-free people from the Swedish general population.
While the team revealed a clear link between antibiotic use and cancer in the ascending colon; They found no such association with cancers in any part of the distal colon or rectum.
ascending colon cancer
The team wanted to determine why the drugs caused cancer in the proximal colon (the ascending colon). To do so, they searched the prescription drug registry for methenamine hippurate, a drug that helps prevent urinary tract infections in people who get them frequently.
Harled explained that despite its antibacterial effect, However, the drug does not alter the gut microbiome because it can only be activated by high acidity of the urine.
Based on the theory that antibiotics increase the risk of cancer by messing with gut bacteria; Methenamine hippurate should not be associated with the same increased risk.
Filtering all of their data, the team found this to be the case: Only antibiotics that affect gut bacteria, not methenamine hippurate, have been shown to be associated with colon cancer.
These findings support the link between antibiotics and cancer, but the study still faces some limitations.
For example, the data sets included no information about the individuals' diet or smoking habits, all of which can also increase colon cancer risk.
Likewise, researchers were unable to identify patients who might be taking antibiotics for an underlying condition such as inflammatory bowel disease, which is also linked to colon cancer.
In addition, the Swedish Register of Prescribed Medicines provides information on prescriptions, but cannot reflect whether individuals have finished a full course of antibiotics.
But because the study is so large, it "definitely points in the right direction," Harled says.
In a few years, the team hopes to do a larger follow-up study, when more data is available, and they're interested in seeing whether certain subtypes of colon cancer show a stronger association with antibiotics.