Oral Antibiotics and Cancer Risk: Is There A Link?

When we talk about the oral-systemic connection, we’re really talking about a subsystem of the human body itself. And while we frequently think of ourselves as 100% human, that isn’t the full picture.

That’s right. You are a moving ecosphere. Microbial life forms living in the human body outnumber your own cells, in fact; up to 100 trillion in every body, including bacteria, viruses, and protozoa. Each person carries up to five pounds of independent organisms, called their “microbiome,” which dwell mostly within the gut; these symbiotic creatures perform a plethora of functions we are only beginning to fully understand, including critical contributions to digestion, immune function, vitamin production, and even blood coagulation.

Like many other critical discoveries in science and health, the existence and role of the microbiome in human health was not widely recognized or accepted until relatively recently: The 1990s.

Just as a vehicle is not made of oil, coolant, and brake fluid (but can’t run optimally without them), the human vehicle is not made of other types of living material–but appears to need them for optimal performance of many body systems:


* Digestive health 

* Metabolic regulation and weight management

* Heart health

* Neurological health

The Potential Microbiome-Cancer Link

Now researchers are looking in earnest at the delicate balance between our native (beneficial) microbiomes, the devastating toll that antibiotics can take on them, and the possible effect of pharmaceuticals on chronic and long-term conditions. (Breast, lung, colon, and prostate cancer correlations with antibiotic administration have been studied, with initially suggestive but inconsistent results.)

One of the most recent studies examined our microbial partnership via the Clinical Practice Research Datalink in the United Kingdom, specifically searching “big data” for potential links between antibiotic administration and site-specific colorectal cancer risk.

The dataset included prescription histories over an average of eight years for 28,930 patients who were diagnosed with bowel cancer (19,726) and rectal cancer (9254). Their medication histories were compared to a control group of 137,077 patients, matched for age and sex, without such cancers.

The results: In patients whose cumulative (additive) use of antibiotics totalled a relatively short period (16+ days), researchers found a small but significantly increased risk of bowel cancer; for rectal cancer, the exact opposite result occurred. Patients who had taken oral antibiotics for more than 60 days showed a 15% lower risk compared to those patients who had not used antibiotics. 

The study also showed differences depending on what type of antibiotics had been taken; patients with proximal bowel cancer had taken drugs targeting anaerobic bacteria (a common class including penicillin and Keflex) at a greater rate than those who did not develop bowel cancer. No such correlation affected those with rectal cancer.

What’s the Upshot?

All medicine is an art as well as a science; the arrival of enormous computing power, even larger databases, and artificial intelligence is opening up new realms of inquiry into cause and effect that researchers and clinicians could not have conceived of just 25 years ago.

Our level of understanding of the role our microbiome plays in our physiology–how our passenger organisms contribute to our bioprocesses, how they factor into everyday metabolic and systemic illness, how these independent life forms negotiate populations, checks and balances between “beneficial” and “malignant” bacteria–is in its infancy.

But the more we learn, the more we are discovering that when a medication destroys a significant population of our internal ecosphere, the consequences may affect the larger organism–in this case, the human hosting those bacteria. And the sudden loss of balance in gut microbiome populations could very well contribute to disease mechanisms elsewhere in the whole system.

All in all, yet another reason to advocate for responsible antibiotic stewardship, for the sake of individuals as well as population health.

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