Dear Readers,

The strategy of taking a daily low-dose aspirin tablet has been in the news again this week, with a study that seems to suggest it can make older people more likely to fall over. This is disappointing because the trial was set up in the hope that it would reduce the risk of falls.

Aspirin has long been used as a treatment for headaches or fever, but the idea of taking a low dose every day for reducing heart attacks and strokes has also grown. In some cases, it is recommended for people who have had several miscarriages to lower their risk of having a further one. Some researchers also think low-dose aspirin may have other benefits, including reducing cancer and Alzheimer’s disease.

It might seem odd that one drug is supposed to do so many different things, but it starts to make more sense once we consider aspirin’s origins. It is fairly well known that aspirin was first isolated from willow bark, used for treating fever and pain since the 1700s. Less well known is that aspirin, or acetylsalicylic acid, is one of a group of compounds called salicylates that are found naturally in many fruits and vegetables.

The salicylates are some of the thousands of chemicals found in plants that are thought to contribute to the fact that people who eat more fruit and veg live longer and healthier lives. In plants, salicylates help trigger cell death when tissues become diseased or damaged.

It has been claimed that modern fruit and veg contain lower levels of salicylates because today’s crops have reduced natural stresses and only unbruised, perfectly formed specimens tend to reach supermarket shelves. So, perhaps we are all now slightly deficient in salicylates. But there isn’t much experimental evidence to support this view.

What has been better studied is aspirin’s effects on the body. The best known is that it inhibits a group of enzymes involved in inflammation called cyclooxygenases (COX), which explains why aspirin reduces pain and swelling from minor injuries as well as fever.

There are two groups of these enzymes, COX-1 and COX-2. The COX-2 enzymes are involved in inflammation. The COX-1 enzymes are involved in reducing the clumping together of platelets, tiny fragments of cells that are normally suspended in the blood. When we cut ourselves, platelets become stickier and form clots that help seal the wound.

Taking daily low-dose aspirin makes platelets slightly less liable to clump together and form clots – this is sometimes described as “thinning” the blood. Because heart attacks and strokes can be triggered by a blood clot blocking circulation to the heart or the brain respectively, it makes sense that they can be reduced by aspirin.

Aspirin is also recommended for some people who have had several miscarriages, but it should only be taken for this reason on a doctor’s advice. It may prevent miscarriages by thinning the blood.

There are downsides to long-term aspirin use, though. People can bleed and bruise more easily because their blood becomes less likely to clot. If bleeding happens in the stomach, particularly from an ulcer, this can be life-threatening, and aspirin can also cause stomach ulcers to form.

The risk is very low, but the benefits in terms of reductions in heart attacks and strokes are also low, therefore much space in medical journals has been devoted to weighing up the risks and benefits for different groups of people.

About 20 years ago, there was serious debate about whether pretty much all adults should take low-dose aspirin, unless they were allergic to it or had a history of ulcers. It has even been investigated whether aspirin should be part of a heart-protective “polypill”, combining five drugs in one tablet. The other components would be a statin for lowering cholesterol and three drugs to lower blood pressure.

These days, the downsides of aspirin in potentially triggering bleeding and stomach ulcers seem to be given stronger weight. In the UK, preventative aspirin is recommended for people thought to be at high risk of heart disease, but not for everyone, and only after discussion with a doctor. Polypills that don’t contain aspirin are being investigated in trials.

How about aspirin’s other potential effects? There are several biological mechanisms through which aspirin could be reducing cancer and Alzheimer’s disease.

Inflammation has now been implicated in so many different conditions, including cancer and Alzheimer’s disease, that aspirin’s anti-inflammatory effects alone could be responsible. It also seems to encourage cancer cells to die by triggering a “self-destruct” programme called apoptosis, as in plants.

Most of these claims don’t stem from the best kind of medical evidence: randomised controlled trials. The ideas about the mechanism of action either come from cells grown in a dish or animal studies – which don’t necessarily translate to humans.

There are many observational studies in people that seem to show benefits against cancer, for instance, but these are subject to the “healthy user bias”. People who take low-dose aspirin are more likely to be health-conscious and better off financially, which are both factors that could slightly reduce people’s likelihood of developing cancer and Alzheimer’s disease.

There has also been trial that looks at aspirin for fall prevention, the idea was that if the drug reduces cognitive decline, it might make people less likely to fall over. And because aspirin is also theorised to help strengthen bones, it could reduce the consequences of falling over too.

Sadly, it appears to do neither, although one of the authors of the study speculates that aspirin might not actually have caused more falls, but that its effect on blood thinning might have meant people bled and bruised more, which made them more likely to be taken to hospital if they had fallen.

For now, the jury is still out on the non-cardiovascular benefits of aspirin. But I will leave you with one last piece of health advice for anyone who is in the unfortunate position of being in the middle of a heart attack.

While you wait for the ambulance to arrive, the advice is that you should take a normal pain-relieving aspirin tablet – in other words, the higher-dose version – in the hope this will help break up any blood clots. You are supposed to chew up the tablet, rather than gulping it down, to help speed its absorption into the blood through the mouth, if anyone can remember to do such a thing while in these dire straits.

• A NewScientist report