Just suppose, for a moment, you are interested enough in medical treatments to list the greatest achievements of the last century.

Naturally you will think of penicillin and all the other antibiotics that have made such an impact on infectious diseases. You would add vaccines to the list, too, as smallpox, diphtheria, measles, whooping cough, and polio are no longer everyday diseases to frighten everyone.

Then there are all the anti-cancer treatments that have turned cancer from a terrifying diagnosis into something to be tackled with optimism, and often cured. In recent years we have hugely reduced deaths from heart attacks and strokes with drugs to prevent them, and better treatments of them when they happen.

Doctors in training no longer have to learn the seven different ways of removing a stomach riddled with ulcers because we now have effective drugs to cure them. We now have asthma treatments that really work without the risk of killing the patient when we apply them - adrenaline injections were our only option for severe asthma attacks 40 years ago, and they frightened the doctors giving them as much as the patients.

Today someone breaking a leg is pinned and walking within a day: no longer do we see rows and rows of people on traction spending weeks in hospital, and getting deep vein thromboses for their pains. Transplants of kidneys, livers, lungs and hearts are commonplace, allowing millions of people all over the world to live years longer than before.

So before you decry the modern health services, think of the advances we have put into practice today, of which we could only dream a few years ago, and be thankful.

Yet amid all these advances there's a drug developed in the 19th century that still astonishes us. It's probably the best-known medicine that we all have swallowed at some time. Its proper name is acetylsalicylic acid: we all recognize it as aspirin.

I don't know of any other drug more than 100 years old that still attracts research papers in the medical journal. Yet last week it featured in both the British Medical Journal and the Lancet as the subject of research into its new uses.

We family doctors have been waiting for both reports for several years. They are the culmination of two studies. In one the researchers were following the use of a single adult aspirin (300mg, not the 75mg dose given to patients with heart problems to prevent clots) every day to prevent bowel cancer. In the other, they were assessing the effect of aspirin on the brain of elderly women, to see if it could prevent loss of intellect.

Why did they think aspirin could have these effects? After all its reputation was built on its being a simple painkiller. But it's much more than that.

For nearly 40 years we have known that it shuts down a particular chemical pathway in the body that promotes the process of inflammation. We think that inflammation is part of the problem in the ageing brain, and damping it down may well help to prevent its deterioration.

Thirty years ago we found that the process that produces cancer of the bowel in people susceptible to it (with several relatives suffering from it) is the same pathway that aspirin cuts across. So it seemed possible that aspirin might prevent the cancers from developing.

The results in the brain weren't as clear-cut as was hoped for. The problem wasn't that it didn't work, however. It was mainly that the subjects in the study - elderly women - didn't show much deterioration in their brainpower regardless of the treatment they had.

Nor were the methods used to measure their brain loss accurate enough to detect early dementia. And the women were so well looked after in other ways (their blood pressures were controlled well, they were encouraged to follow a good lifestyle) that they were really a special group unlike the population at large.

So the jury is out on taking aspirin every day to prevent dementia. But it very definitely came to an astonishing conclusion on bowel cancer. It was given to people with a known risk of developing bowel cancer (they had adenomas' - precancerous tumours).

There was a 70 percent reduction in cancers in those taking daily aspirin for more than five years, providing the dose was 300mg or more. The reduction wasn't apparent until ten years after the start of the trials, because in the first decade cancers had obviously already developed in some patients, and these could not have been prevented. From the tenth year onwards, the aspirin effect certainly kicked in.

What can we learn from this? If you have bowel cancer in your family you should ask your doctor about your risk. If it is above normal, you will be asked to take aspirin. The modern aspirin tablets (clear or fizzy) are much less likely to upset your stomach than the older ones, but if you have had a reaction in the past to it, you need to talk about that, too. It is clear, though, that for many people a single adult aspirin a day will eventually save their lives.