A news topic on "pain relievers increase the risk of breast cancer" has been covered in multiple news agencies in the past several days. This comes as a shock since anti-inflammatory pain relievers such as Aspirin were believed to lower the risks of inflammatory disorders such as breast cancer, IBD, etc.
The findings were published in the May 31st 2005 issue of Journal of the National Cancer Institute. The study followed 114,460 women who participated in a previous California Teachers Study (so I suppose they are women, teachers and Californians) for 6 years. At the beginning of the study, they were all free of breast cancer (not necessary free of other cancers or inflammatory disorders). During 1995-2001, 2391 (2.1%) women were diagnosed with breast cancer. Data on the use, frequency and duration of nonsteroidal anti-inflammatory drugs (NSAIDS) was collected through a self-administered questionnaires (1995, 1997 and 2000).
Most news coverage of this study only cited the above description of the study. Then the news would go on saying that "daily long-term use of aspirin was associated with an 81% increased risk" (RR=1.81, 95% CI= 1.12 to 2.92).
First, what is an 81% increase in risk when the overall risk is only 2%?
In the paper, the authors honestly presented the following findings, which is no surprise at all:
"After accounting for age, regular NSAID users were more likely to be white, to be overweight or obese, to be current or former smokers, to have had a mammogram in the last 2 years, and to have used postmenopausal hormone therapy than regular NSAID users. "
In their multi-way tables that present the relative risk estimates of NSAID use, it is not "crystal clear" how exactly the statistical analysis was done. However, it does mention the estimation adjusted (by including the covariates on) race, BMI, first-degree family breast cancer history, menopausal and hormone therapy use, smoking, alcohol intake, physical activities, mammography history, breast biopsy history, parity status before age 30 and neighborhood socioeconomic status.
Is there something missing? I was looking for age. That would have been my first guess. Age should matter most. Who will have higher risk of cancer? And who will be more likely to be regular pain reliever users? But I can't seem to find a mention of it in my 10-minuet quick browse of their paper. I am sure the investigators have considered age as important. In another table, it is mentioned that the characteristics are age-standardized (well, meaning that they standardized the values of these covariates within each 10-year age category). This will make the other covariates not heavily correlated with age. But age is not adjusted for in estimating relative risks? Is this a typo or an unfortunate omission?
There are some other statistical question marks in this paper that I didn't figure out during my fast read. Not to mention multiple comparison issues.