Patients weighing more than 70kg should be advised to consider doubling their aspirin doses, or told to take a low dose twice daily, researchers have said.
The new analysis from the University of Oxford found that lower doses of aspirin were only effective in preventing vascular events in patients weighing less than 70kg.
Researchers found that the ability of 75-100mg aspirin to reduce cardiovascular events decreased with increasing weight, with those weighing 50-69kg seeing a 25% reduced risk compared to those not on aspirin. No benefit was seen in 80% of men and nearly 50% of women weighing 70kg or more, with only a 5% reduction in risk compared to those not on aspirin.
Conversely, higher doses of aspirin (≥325mg) had the opposite effect with bodyweight, reducing cardiovascular events only at higher weights.
Doses of 325mg reduced the risk of events by 17% in patients weighing more than 70kg, compared to patients not on aspirin. Doses of 500mg reduced the risk by 45% in patients weighing 90kg or more compared to non-users.
These findings were consistent across both men and women, in those with diabetes, and in relation to height.
The research looked at randomised trials of aspirin in primary and secondary prevention for cardiovascular events and stroke to analyse the effect of bodyweight and height on the impact of low doses (≤100mg) and higher doses (300-325mg or ≥500mg) of aspirin.
The researchers wrote: ‘A one-dose-fits-all approach to use of aspirin has yielded only modest benefits in long-term prevention of cardiovascular events, possibly due to underdosing in patients of large body size and excess dosing in patients of small body size, which might also affect other outcomes.’
They continued: ‘The substantial reductions in cardiovascular events and death at optimal doses for weight highlight the potential to improve effectiveness and argue for a more tailored dosing strategy.’
Lead researcher Professor Peter Rothwell, director of the Centre for Prevention of Stroke and Dementia at the University of Oxford, said: ‘The finding of loss of efficacy of low-dose (75-100mg) enteric-coated aspirin at bodyweight over 70kg is of most clinical relevance in the secondary prevention setting.
‘It might be prudent to either double the dose in patients at higher weight or to take low-dose aspirin twice daily.’