For more than a headache

FOR thousands of years aspirin has been humanity's wonder drug. Extracts from the willow tree have been used for pain relief in folk medicine since the time of the ancient Greeks. By 1897 a synthetic derivative (acetyl salicylic acid) of the plant's active ingredient (salicin) was created. This allowed aspirin to become the most widely used medicine in the world.

In recent years its benefits as a blood-thinning drug have led to it being prescribed in low doses of around 50mg to reduce deaths from stroke and heart attack. There were also hints that aspirin may help prevent some cancers. But these were mostly based on observational studies, which can be misleading.

The gold standard of scientific evidence is the randomised controlled trial, preferably one with a lot of people and held over a long time. The results of just such a trial, published in the Lancet, suggest that aspirin is indeed an astonishing drug. Peter Rothwell at the John Radcliffe Hospital in Oxford and his colleagues looked at deaths due to cancers during and after randomised trials of daily aspirin. The trials had actually been started to look at how useful aspirin was for preventing heart attacks and strokes. Nevertheless, the data from the 25,570 patients enrolled in eight trials was also revealing about cancer.

In trials lasting between four and eight years, the patients who had been given aspirin were 21% less likely to die from cancer than those who had been given a placebo. These results were based on 674 cancer deaths, so are unlikely to represent the kind of statistical oddity that can beset studies on cancer risks that sometimes create headlines.

The benefits of aspirin were also apparent many years after the trials had ended. After five years, death rates for all cancers fell by 35% and for gastrointestinal cancers by 54%. A long-term follow-up of patients showed that the 20-year risk of cancer death remained 20% lower in those who had taken aspirin.

The study revealed that the effect takes time to accrue, so aspirin must be taken over a long period. The latent period for improving oesophageal, pancreatic, brain and lung cancer was about five years of aspirin taking on a daily basis. For stomach and colorectal cancer the effects took ten years and for prostate cancer about 15 years. The means by which aspirin prevents cancer is not well understood. It is believed that it inhibits an enzyme that promotes cell proliferation in tumours.

The researchers also found that small daily doses of aspirin were enough, and that taking more than 75mg conferred no additional benefits. Those starting on aspirin in their late 40s or 50s benefit most.

Current guidelines on using aspirin for reducing the chances of a stroke or heart attack rightly warn of the small risk of ulcers and of dangerous bleeding in the stomach. These guidelines will probably have to be revised given the new findings. However, it remains unlikely that popping aspirin will be recommended for everyone like a vitamin supplement.

Aspirin is a highly cost-effective treatment: taking it for five to ten years easily beats initiatives to screen for breast and prostate cancers. To put it another way, ask yourself what a pharmaceuticals firm might charge for a drug that would reduce the chance of death by cancer by 20%—and then note that 100 days' supply of low-dose aspirin can cost less than a dollar. By anyone's measure, that is a bargain.