What is aspirin and what does it do in your body?

Professor Warner talks about aspirin

People with cardiovascular disease may be prescribed a low dose of aspirin to help prevent blood clots. Doireann Maddock puts common questions about them to Professor Tim Warner, Professor of Vascular Inflammation at Barts and the London.

How does aspirin work?

The way in which aspirin works was discovered by the British researcher Sir John Vane about 40 years ago. He showed that aspirin blocks an enzyme called cyclooxygenase. Cyclooxygenase produces a range of hormones known as prostanoids. In the blood, these prostanoids cause platelets to stick together, so blocking cyclooxygenase with aspirin reduces the formation of blood clots. Prostanoids also affect inflamed areas of the body, such as sprains and damaged joints. This is why in the past aspirin was used as a painkiller and anti-inflammatory drug. However, much higher doses are needed to inhibit pain and reduce inflammation than to reduce the risk of blood clots, and we now use non-steroidal anti-inflammatory drugs such as ibuprofen, which are better at this.

Why have I been prescribed aspirin?


“It’s vital that you discuss with your doctor or pharmacist before taking any additional medication”


Typically, a daily, low dose of aspirin (75mg) is prescribed for people who have a cardiovascular disease, such as peripheral arterial disease, or who have had a heart attack or certain types of stroke. It’s prescribed in this form to reduce the risk of blood clots forming (we call this thinning the blood). What we mean by this is that aspirin can inhibit circulating blood cells known as platelets.

One of the most important protective roles of platelets is to stick together and block cuts and breaks in blood vessels, so they are important in normal health. However, in people at risk of heart attacks and some kinds of stroke, platelets can stick together inside blood vessels causing a clot and stop blood flowing to the heart or brain. Aspirin reduces the chances of this happening, which makes it useful for patients at risk.

Should we all take daily aspirin to prevent blood clots?

Current thinking doesn’t recommend this. If you have a cardiovascular disease, particularly if you have had a heart attack or stroke, studies have shown that the benefits of taking aspirin far outweigh the risk of side effects. But for people without a cardiovascular disease, the risk of side effects outweighs the benefit of preventing blood clots.

What are the possible side effects of aspirin?

“The risk of side effects is why aspirin is not recommended to be taken by otherwise healthy people”

The most likely side effect is an increased risk of bleeding because aspirin reduces the risk of platelets sticking together. Generally, these bleeds are very minor, but occasionally patients may experience severe bleeds. These are most commonly in the stomach or intestines, where they can cause substantial and occasionally life-threatening blood loss. They are more likely to happen if you have a stomach or duodenal ulcer. Symptoms of this type of severe bleed include upper abdominal pain, passing blood or black stools, or vomiting blood. If this happens, seek urgent medical attention.

Severe bleeds may also occur in the brain, leading to a stroke. These rare, but potentially very dangerous and severe, side effects explain why aspirin is not recommended to be taken by otherwise healthy people. Occasionally, people who have asthma find they are sensitive to aspirin and that it can cause an asthmatic attack. Anyone who suffers from asthma should avoid aspirin and other non-steroidal anti-inflammatory drugs such as ibuprofen.

Is there a reason why my GP wouldn’t prescribe me aspirin?

The most likely reason is because you have previously been found to be sensitive to the asthmatic effects of aspirin or other non-steroidal anti-inflammatory drugs. Another reason may be that you have a medical history of bleeding in the stomach or intestines. Your doctor would consider the balance of benefits versus risks in situations such as this.

What’s the alternative to aspirin?

People who are advised to take an aspirin by their doctor but can’t, most commonly because they are sensitive asthmatics, can sometimes take the drug clopidogrel. This drug inhibits the blood platelets but doesn’t affect cyclooxygenase.

Are there any foods that I should avoid?

There are no foods that you specifically need to avoid if you are taking aspirin.

What if I have a pain and want to take another type of painkiller?

The general recommendation would be to try paracetamol first of all. Avoid high doses of nonsteroidal anti-inflammatory drugs such as ibuprofen without first discussing with your doctor, as the combination with aspirin increases the risk of bleeding.

Are there any other medications I should avoid if I’m taking aspirin?

As with all drugs, it’s vital that you discuss with your doctor or pharmacist before taking any additional medication.

Published on the BHF website

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