Background: Randomized trials show non-steroidal anti-inflammatory drugs (NSAIDs) reduce precancerous polyps. Observational studies of the NSAID aspirin (ASA) suggest that it reduces invasive colorectal cancer (CRC) incidence, but because ASA use may also be a marker for healthy behaviors, these studies may be subject to selection bias. We sought to estimate the effectiveness of NSAIDs in CRC prevention in the population of elderly Medicare beneficiaries, minimizing this selection bias.
Methods: With National Ambulatory Medical Care Survey data, we find that patients with a diagnosis of osteoarthritis (OA) are 4.4 times more likely to concurrently have NSAID use documented than patients without such a diagnosis. We use this figure to estimate the expected NSAID-mediated reduction in CRC risk associated with a diagnosis of OA. Using Survival Epidemiology and End-Results (SEER)-Medicare data, we compare cases of elderly Medicare beneficiaries diagnosed with CRC in 1995 to persons without CRC to determine if their odds of antecedent OA differ.
Results: We estimate the expected NSAID-mediated reduction in CRC associated with an OA diagnosis to be between 6 and 16% (i.e., RR, 0.84-0.94). In the SEER-Medicare data, we find that individuals with a diagnosis of OA in Medicare claims in the previous 3 years had 15% lower odds of being diagnosed with CRC than individuals whose claims did not reflect antecedent OA (OR 0.85, 95%CI 0.80-0.91).
Conclusions: This case-control study finds that elderly Medicare beneficiaries with histories of OA have 15% lower odds of developing CRC. These results are consistent with a preventive role for NSAIDs in CRC among the elderly.