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Low-dose aspirin only effective in patients weighing less than 70kg, say researchers

GPs should consider doubling the aspirin doses in patients weighing more than 70kg, or advise them to take a low dose twice daily, researchers have said.

The new study from the University of Oxford found that lower doses of aspirin were only effective in preventing vascular events in patients weighing less than 70kg, with no benefit in 80% of men and nearly 50% of women weighing more.

NICE has said that any new evidence relating to aspirin dose is 'likely to be considered' in the acute coronary syndromes guidance currently being updated, but GPs have stressed that the value of aspirin for secondary prevention 'must not be lost' in these findings.

The research, published in The Lancet, analysed the effect of bodyweight and height on the impact of low doses (≤100 mg) and higher doses (300–325mg or ≥500 mg) of aspirin, using randomised trials of aspirin in primary and secondary prevention for cardiovascular events and stroke.

Researchers found that the ability of 75–100mg aspirin to reduce cardiovascular events decreased with increasing weight, with those weighing 50–69kg seeing a benefit, while no benefit was seen in 80% of men and nearly 50% of women weighing 70kg or more.

Conversely, higher doses of aspirin (≥325mg) had the opposite effect with bodyweight, reducing cardiovascular events only at higher weights.

These findings were consistent across both men and women, in those with diabetes, and in relation to height.

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.’

A NICE spokesperson told Pulse: ‘We’re currently updating our guidelines covering acute coronary syndromes and this will include recommendations on the initial choice of antiplatelet agents for adults with unstable angina, NSTEMI (non-ST-elevation myocardial infarction) or STEMI (ST-Elevation Myocardial Infarction).

'The updated guideline is due to be published in May 2020 and so any new evidence relating to aspirin dose is likely to be considered during its development.’

They added that the guideline on myocardial infarction: cardiac rehabilitation and prevention of further cardiovascular disease does not recommend a specific dose of aspirin.

GP with special interest in cardiology Dr Ivan Benett said: ‘The message that must not be lost is that aspirin is very valuable for secondary prevention, and the message should not be confused with this indication.

‘However, this study does raise the question as to whether there should be a prospective trial of primary prevention with aspirin doses titrated against weight. Indeed it may also raise the question about aspirin doses in secondary prevention.’

Earlier this year, research found that patients could be switched to aspirin as a cheaper alternative for treating venous thromboembolism after hip and knee replacements, as it does not increase the risk of blood clots.

Readers' comments (6)

  • How frustrating! We advise people based on the dogma of the time, but when new evidence comes along we ditch the previous incotravertable proof. We must constantly remind ourselves as a profession we don't know everything and we should stop pretending that we do.

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  • who uses enteric coated aspirin?
    Perhaps they should have studied dispersible. Might not make a difference, but who knows?

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  • AlanAlmond

    Is that 70kg with shoes or without?

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  • Vinci Ho

    Whenever a theory appears to you as the only possible one, take this as a sign that you have neither understood the theory nor the problem which it was intended to solve.
    Karl Popper

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  • Bob Hodges

    Good opportunity to review and stop a lot of 75mg prescriptions for primary prevention only........fewer prescriptions = fewer requests at reception = less work = less chance of a cock-up.........and possibly fewer GI bleeds.

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  • aspirin not used for primary prevention anymore is is?

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