Dubious antibiotic cancer link

Antibiotic use increases breast cancer risk, according to a new study. But after scrutinizing the study, I’m leaning toward linking grant-hungry researchers and a publicity hungry medical journal with reprehensible sensationalism.

The study in the Feb. 18 issue of the Journal of the American Medical Association reported all levels of antibiotic use were associated with increased risk of breast cancer and death from breast cancer.

The study triggered an avalanche of “Antibiotics Linked to Breast Cancer” news reports, most of which were sensibly tempered with “don’t panic,” “don’t stop taking antibiotics” and “more research is needed” caveats.

Researchers compared antibiotic use among 2,266 women with breast cancer and 7,953 women without breast cancer, all of whom belonged to a large health plan in western Washington State. Data on antibiotic use was obtained from the health plan’s records.

Antibiotic use for 1 to 50 cumulative days reportedly was associated with a 50 percent increase in breast cancer risk. Antibiotic use for more than 1,000 cumulative days reportedly was associated with a 100 percent increase in risk. The results sound scary, but here’s why they’re not.

No evidence was presented that antibiotics were the biological cause of any of the cases of breast cancer considered in the study. This is no surprise since no demonstrable biological explanation exists as to why antibiotics would cause cancer in the first place.

Without a plausible biological link between antibiotic use and breast cancer, the researchers relied exclusively on statistical analysis, a potentially useful tool if the data analyzed are of reasonable quality. These study data, however, fall way short.

The average study subject was about 60 years old. But study subjects who had taken antibiotics had been enrolled in the health plan for only about 20 years on average. Since the sole source for data on antibiotic use was the health plan’s records, about 40 years of data, on average, about potential antibiotic use were missing for each study subject.

Even more data are missing for the 18 percent of the study subjects who supposedly never took antibiotics. These women had been in the health plan for only about 10 years on average.

But the mere fact they didn’t take antibiotics while enrolled in the health plan doesn’t mean they didn’t take antibiotics before enrollment. Indeed, some of the supposed “never users” could actually have been extremely heavy users of antibiotics prior to enrollment in the health plan.

Since the vast majority of Americans have taken antibiotics at some point, it’s difficult to believe so many of the study subjects had never taken antibiotics.

This is a crucial data gap, since the researchers claim even a single day of antibiotic use increased breast cancer risk. The absence of complete data on lifetime antibiotic use renders comparisons between users and “nonusers” meaningless.

The study data are also faulty in terms of level of exposure to antibiotics. The researchers assumed exposure to antibiotics could be measured either by number of antibiotic prescriptions written or by the number of days prescribed for antibiotic use according to prescription records.

But patients commonly fail to complete courses of antibiotics prescribed by their doctors. Patients with a prescription for 10 days of antibiotics may feel better after just a few days and cease taking their medicine. A 10-day prescription, therefore, doesn’t necessarily mean 10 days of use. It may, in fact, mean much less use.

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