Increased breast cancer risk for patients with multiple sclerosis: a nationwide population‐based cohort study

Background and purpose Studies have suggested that multiple sclerosis ( MS ) might be linked to an overall reduced cancer rate, but a positive relationship is also found for several types of cancer. This study determines whether MS is associated with cancer risk in T aiwan. Methods Data from the N ational H ealth I nsurance S ystem of T aiwan were used to assess this issue. The MS cohort included 1292 patients, and each patient was randomly frequency‐matched with four participants without MS , based on their age, sex and index year (control cohort). C ox proportional hazards regression analysis was conducted to estimate the influence of MS on cancer risk. Results A significantly higher risk of developing overall cancer was found amongst the MS cohort compared with the control cohort [adjusted hazard ratio ( HR ) 1.85, 95% confidence interval ( CI ) 1.26–2.74], as well as breast cancer (adjusted HR  2.23, 95% CI  1.11–4.46). Conclusions The nationwide population‐based cohort study revealed that T aiwanese patients with MS have a higher risk of developing overall cancer types and breast cancer in particular.