Long-term use of low-dose aspirin for cancer prevention: A 20-year longitudinal cohort study of 1,506,525 Hong Kong residents.

Long-term use of low-dose aspirin has been demonstrated to reduce cancer risk, but the duration of necessary medication use remains uncertain. This study aimed to investigate the long-term chemoprotective effect of aspirin among the Chinese population. This population-based study included all aspirin users between 2000 and 2019. Aspirin users were age-sex matched with non-users at a 1:2 ratio. Cancer incidence and mortality were the main outcomes measured. Survival analyses with the Fine-Gray modelling were performed. The chemoprotective effects were measured by the sub-distribution hazard ratios (SHR) with control for the competing risks. A total of 538,147 aspirin users and 968,378 non-users were included, with a mean age of 64.8 years, 9,543,399 person-years of follow-up and 90% of users with 80 mg aspirin. The long-term use of aspirin was associated with a reduced risk of cancer (SHR 0.92, 95% CI 0.91-0.94) and a reduced risk of cancer mortality (SHR 0.80, 95% CI 0.79-0.82). Stronger chemopreventive effects were observed among those who used aspirin for more than 10 years, including risk reductions for lung (SHR 0.56, 95% CI 0.51-0.60), breast (SHR 0.34, 95% CI 0.29-0.38) and colorectal (SHR 0.37, 95% CI 0.33-0.40) cancers, but not for bladder cancer and leukaemia. Low-dose use of aspirin was associated with lower risk of cancer among Chinese. The association was even stronger for those using aspirin for more than 10 years. Prescription of aspirin may be started as early as at age of 40, as the chemoprotective effect also applied for early cancers.