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We know that that bacteria can have an important part in maintaining healthy bowel function. More recently a number have studies have looked at the importance of gut or bowel bacteria in maintaining the immune system in the bowel, which is vital to fight infections and prevent cancer. More recently, it seems that gut bacteria may influence the whole body’s immune system and may also have an impact on altering the risk of developing colon cancer within the colon.

It is theoretically possible that overuse/long-term use of antibiotics may allow harmful bacteria to proliferate within the bowel, which might ultimately influence immune function and the risk of developing cancer.

Several population studies have identified an association between antibiotic use and an increased risk of cancer. Some studies have suggested that increased use of long-term antibiotics in early to middle adulthood can lead to an increased risk of developing precancerous polyps.

Researchers have recently published data from the United Kingdom which compared the risk of bowel cancer in groups who had been prescribed different amounts of antibiotics.

The researchers compared the use of antibiotics in 29,000 patients who developed bowel (colorectal) cancer with 137,000 patients who did not have bowel cancer. Although the impact of antibiotic use was very small, the results did suggest that antibiotic use was associated with an increased risk of colon cancer.

It was found that antibiotics had previously been prescribed in 71.5% of patients who developed bowel cancer versus 69% of those who did not develop bowel cancer. The increased risk of colon cancer was not an immediate effect and the slightly increased risk was only apparent in those who had been given antibiotics at least 10 years before the diagnosis of bowel cancer i.e. it would appear to take at least 10 years before it has an impact.

Specifically, the risk of colon cancer was increased with regular/prolonged use of antibiotics (16+ days) that affected certain bacteria (anaerobic bacteria) compared to those who has not been given any antibiotics.

Interestingly, there was an inverse relationship between the use of antibiotics and the development of rectal cancer i.e. antibiotic use appeared to reduce the risk of rectal cancer. O the data suggested that antibiotic use for more than 60 days was associated with a 15% reduced risk of rectal cancer compared to no antibiotic use. In particular, one antibiotic (tetracycline) was associated with a reduced risk of rectal cancer whereas ampicillin/amoxicillin appeared to increase the risk of colon cancer.

Population studies have also suggested that early prolonged use of antibiotics is associated with an increased risk of the development of adenomas in the left or proximal colon but not in the rectum.

Although this current study does suggest that there may be an association between the use of antibiotics and the subsequent development of bowel cancer, it was not able to completely exclude the impact of other issues such as a high BMI, smoking and alcohol use which we know also increase the risk of bowel cancer. It may be that some of the perceived increased risk of colon cancer with antibiotic use is actually due to other linked factors e.g. patients who smoke have an increased risk of bowel cancer. However, smoking may also lead to more chest infections and increased use of antibiotics, thereby giving the impression that the antibiotic is increasing the risk of bowel cancer whereas in fact, it is the smoking that truly increases the risk of bowel cancer.

It is also important to remember that antibiotics can be life-saving for many patients and there may be no other option other than prescribing prolonged courses of antibiotics.

However, this study reinforces the need to be careful when prescribing antibiotics.


Zhang J, Haines C, Watson AJM, et al

Oral antibiotic use and risk of colorectal cancer in the United Kingdom, 1989–2012: a matched case–control study

Gut Published Online First: 19 August 2019. doi: 10.1136/gutjnl-2019-318593

Antibiotics & the risk of bowel cancer (August 2019)

Antibiotics and risk of bowel cancer