Parallel analyses were performed for aspirin and acetaminophen. We tested the proportional hazards assumption by
including time varying covariates which represented the interactions between each level of aspirin use and time. We assessed effect modification by including all two-way interaction (product) terms between level of aspirin use and
other covariates and using likelihood ratio tests of their combined statistical significance with a = 0.05. Multivariate analyses used standard statistical programs (PHREG; SAS. Cary, NC).