Three studies published in The Lancet journals and including data from 51 RCTs show that a patient's risk of death from cancer falls by 37% if they take aspirin for five years or more.

The analyses suggest that if 1,000 patients took aspirin each year, three fewer cases of cancer would develop.

Academics warned that although aspirin prophylaxis to prevent cancer can no longer be ignored, the effect of major bleeds on quality of life remains unaccounted for.

Evidence limited
Previous research had suggested aspirin might prevent cancer, particularly gastrointestinal cancers, and reduce cancer mortality.

But there is limited evidence for a reduction in incidence, researchers said, and trials have focused on men rather than women. Furthermore, the balance between reduced risk of cancer and increased chance of internal bleeding with extended use has yet to be established, they said.

In the first of three studies, researchers looked at data from 51 RCTs that compared daily aspirin against no aspirin to prevent vascular events.

Aspirin was found to reduce the risk of cancer death by 15% among 69,224 participants in these trials, rising to 37% after five years of daily aspirin.

The absolute risk of cancer death fell from 12 per 1,000 patients not taking aspirin to nine per 1,000 patients on daily aspirin.

Authors said: 'Alongside the previously reported reduction by aspirin of the long-term risk of cancer death, the short-term reductions in cancer incidence and mortality and the decrease in risk of major extracranial bleeds with extended use, and their low case-fatality, add to the case for daily aspirin in prevention of cancer.'

They added: 'In view of the very low rates of vascular events in recent and ongoing trials of aspirin in primary prevention, prevention of cancer could become the main justification for aspirin use in this setting.'

Effect on metastasis

In a second article, researchers found that after over six years on aspirin, the risk of developing adenocarcinoma with metastasis at initial diagnosis fell by 31%.

The risk of metastasis developing subsequently among patients without initial metastasis also fell by 55% - most markedly in patients with colorectal cancer, by 74%.

In a third study, researchers further examined aspirin's effect on metastases. Observational studies suggested a 38% reduced risk of colorectal cancer developing initially, and a 31% fall in proportion of cancers with distant metastases.

Warning over bleeds
But in a comment, experts warned that the risk of major bleeding remained unaccounted for.

Dr Andrew Chan and Dr Nancy Cook of Harvard Medical School in Boston, said: 'Despite a convincing case that the vascular and anticancer benefits of aspirin outweigh the harms of major extracranial bleeding, these analyses do not account for less serious adverse effects on quality of life, such as less severe bleeding.'

Nevertheless, the analysis 'moves us another step closer to broadening recommendations for aspirin use', they said.

'Moreover, future evidence-based guidelines for aspirin prophylaxis can no longer consider the use of aspirin for the prevention of vascular disease in isolation from cancer prevention.'