A short-term risk–benefit analysis of occasional and regular use of low-dose aspirin in primary prevention of vascular diseases: a nationwide population-based study

OBJECTIVE: To calculate the short-term risk–benefit effect of occasional and regular use of low-dose aspirin (≤100 mg/day) in primary prevention. STUDY DESIGN: Two retrospective cohort studies. SETTING: Taiwan. PARTICIPANTS: 63 788 and 24 910 patients of two nationwide population-based studies were examined. METHODS: Two databases of 1 000 000 patients were randomly sampled from data of Taiwan's National Health Insurance (NHI) for years 1997–2000 (NHI 2000) and 2005 (NHI 2005). In NHI 2000, 63 788 patients 30–95 years of age were found not to have previously been prescribed aspirin before 1 January 2000, but to have first been prescribed low-dose aspirin after that date. They were also found to be at risk of first hospitalisation for any major vascular diseases including haemorrhage (major gastrointestinal haemorrhage or cerebral haemorrhage) and ischaemia (acute myocardial infarction or ischaemic stroke) after their first prescription. We also applied it to NHI 2005, and the number of eligible patients was 24 910. Patients prescribed low-dose aspirin for <20% of the days of a 60-day follow-up period were considered to be occasional users, and those prescribed low-dose aspirin for ≥80% of the days were considered to be regular users. Differences in rate of haemorrhage and ischaemia between these users were used to calculate their net clinical risk. PRIMARY OUTCOME: Vascular diseases. RESULTS: In NHI 2000, the overall unadjusted rates of haemorrhage and ischaemia were 0.09% and 0.21%, respectively, for occasional users and 0.32% and 2.30%, respectively, for regular users. Adjusted net clinical risk of low-dose aspirin use between the two groups was 2.24% (95% CI 2.03% to 2.48%; p<0.001). Similar results were also found in NHI 2005. CONCLUSIONS: Short-term regular use of low-dose aspirin might not be better than occasional use for preventing major vascular diseases in primary prevention. Prescribing regular low-dose aspirin for primary prevention should be done with caution. Future studies should explore the risk–benefit effect of long-term low-dose aspirin use in primary prevention.