Aspirin (Micropirin)

How does it work?

Micropirin enteric coated tablets contain the active ingredient acetylsalicylic acid, otherwise known as aspirin. (NB. Aspirin is also available without a brand name, ie as the generic medicine.)

Aspirin belongs to a group of medicines called non-steroidal anti-inflammatory drugs (NSAIDs). It works by blocking the action of a substance in the body called cyclo-oxygenase.

Cyclo-oxygenase is involved in the production of various chemicals in the body. These are known as prostaglandins, prostacyclins and thromboxane. By blocking the action of cylo-oxygenase, aspirin prevents the production of these chemicals.

Aspirin prevents blood cells called platelets from producing thromboxane. Thromboxane is one of the chemicals that causes platelets to clump together and start off the process of blood clotting. Stopping its production reduces the likelihood of clots forming in the blood.

A blood clot that forms inside the blood vessels is known as a thrombosis and can be dangerous, as it can cause a blockage that cuts off the blood supply to an organ. A blockage in the arteries supplying blood to the heart or brain can cause a heart attack or stroke.

Blood clots and blockages mainly result from a build up of atherosclerosis on the inside of blood vessels. Atherosclerosis is a build up of cholesterol and fat cells that narrows the blood vessels and makes their interiors rough and bumpy. This makes it more difficult for blood to flow through the vessels, and increases the likelihood of clots forming in the vessels. People with angina, or who have already suffered a heart attack or stroke, are at risk of having another because of the atherosclerosis in their arteries. Low dose aspirin is used to lower the risk of this.

High doses of aspirin (300mg and over) also prevent the production of prostaglandins. Prostaglandins are produced in response to injury or certain diseases and would otherwise go on to cause pain, swelling and inflammation. High doses of aspirin are therefore used to relieve pain and inflammation. See the factsheets linked at the end of this article for more information about this use of aspirin.

Aspirin is also used in the emergency situation of a heart attack. Anyone who has the symptoms of a heart attack (chest pain, possibly radiating towards the arm or neck, shortness of breath) should take one 300mg aspirin tablet as soon as possible, as this has been shown to increase the chances of surviving a heart attack. This is because the aspirin prevents the blood clot that is blocking the supply of blood to the heart from growing any bigger. The tablet can be crushed under the tongue for faster absorption into the bloodstream, as the area under the tongue is very rich in blood vessels.

What is it used for?

  • Preventing blood clots in the blood vessels and thus reducing the risk of heart attack and stroke, for example in people with angina, or people who have already had a heart attack, heart bypass, stroke or mini-stroke (TIA).


  • You should consult your doctor if you are considering taking aspirin to reduce the risk of suffering a stroke or heart attack, to make sure it would be a suitable thing for you to do. Aspirin can actually increase the risk of a particular type of stroke in certain people, and it may also have side effects that outweigh its potential benefits for some people.
  • Children under 16 years of age should not take aspirin, unless on the advice of a doctor. This is because aspirin use in children has been associated with a rare condition called Reye's syndrome. This condition affects the brain and liver and though extremely rare, can be fatal. The causes of Reye's syndrome are not fully understood, but use of aspirin to treat fever in children with a virus has been implicated. There are many paracetamol and ibuprofen products not associated with Reye's syndrome available to treat pain and fever in this age group. For more advice talk to your pharmacist.

Use with caution in

  • Elderly people.
  • Decreased kidney function.
  • Decreased liver function.
  • High blood pressure (hypertension).
  • History of breathing difficulties due to a narrowing of the airways (bronchospasm).
  • Asthma.

Not to be used in

  • Children under 16 years of age, unless prescribed by a doctor (see warning above).
  • Bleeding disorders such as haemophilia.
  • Peptic ulcer or history of this.
  • People in whom aspirin or other NSAIDs, eg ibuprofen, cause allergic reactions such as asthma attacks, itchy rash (urticaria), nasal inflammation (rhinitis) or swelling of the lips, tongue and throat (angioedema).
  • Third trimester of pregnancy.
  • Breastfeeding.

This medicine should not be used if you are allergic to one or any of its ingredients. Please inform your doctor or pharmacist if you have previously experienced such an allergy.If you feel you have experienced an allergic reaction, stop using this medicine and inform your doctor or pharmacist immediately.

Pregnancy and breastfeeding

Certain medicines should not be used during pregnancy or breastfeeding. However, other medicines may be safely used in pregnancy or breastfeeding providing the benefits to the mother outweigh the risks to the unborn baby. Always inform your doctor if you are pregnant or planning a pregnancy, before using any medicine.

  • This medicine should be avoided in the third trimester of pregnancy, as it may affect the growth and development of the foetus or have harmful effects on foetal tissues. Seek medical advice from your doctor before using this medicine during any stage of pregnancy.
  • Significant amounts of this medicine may pass into breast milk. It should not be used by breastfeeding mothers as it may be harmful to the nursing infant. Seek medical advice from your doctor.

Label warnings

  • Do not take indigestion remedies at the same time of day as this medication.
  • This medication is to be swallowed whole, not chewed.
  • This medication contains aspirin.

Side effects

Medicines and their possible side effects can affect individual people in different ways. The following are some of the side effects that are known to be associated with this medicine. Just because a side effect is stated here does not mean that all people using this medicine will experience that or any side effect.

  • Indigestion (dyspepsia).
  • Stomach or intestinal discomfort.
  • Worsening of asthma.
  • Allergic reactions such as skin rash, swelling of the lips, tongue and throat (angioedema) or narrowing of the airways (bronchospasm).
  • Ulceration or bleeding in the stomach or intestines.

The side effects listed above may not include all of the side effects reported by the drug's manufacturer.For more information about any other possible risks associated with this medicine, please read the information provided with the medicine or consult your doctor or pharmacist.

How can this medicine affect other medicines?

If you are taking any other medicines you should check with your pharmacist before taking this one to ensure that the combination is safe.

People taking anticoagulant medicines used to prevent the blood clotting, eg warfarin, should not take aspirin to relieve pain or inflammation. This is because the higher doses of aspirin used for pain relief can irritate the stomach lining, as well as increasing the effects of warfarin, both of which increase the likelihood of bleeding. Lower doses of aspirin used for a blood-thinning effect are safer, but should only be used by people taking anticoagulants such as warfarin on the advice of a doctor.

There may be an increased risk of bleeding if aspirin is taken with other 'blood-thinning' (antiplatelet) medicines such as clopidogrel or dipyridamole.

Aspirin reduces the rate at which the body can remove the medicine methotrexate. The two should not usually be used together.

There is an increased risk of side effects if aspirin is taken with other non-steroidal anti-inflammatory drugs (NSAIDs), eg ibuprofen, diclofenac, indometacin. For this reason, aspirin should not be taken with any other NSAID. Low-dose aspirin used for anti-blood-clotting purposes is an exception to this, but should only be used with other NSAIDs on the instruction of a doctor.

There may be an increased risk of bleeding or ulceration of the stomach or intestines if aspirin is taken with corticosteroids, eg prednisolone, dexamethasone.

There may be an increased risk of side effects if aspirin is taken with acetazolamide.

Micropirin tablets have a special 'enteric coating' that is designed to stop the aspirin from irritating the stomach. Antacids (indigestion remedies) should not be taken at the same time of day as Micropirin tablets, because they allow this special coat to dissolve and thus stop it protecting the stomach.

Other medicines containing the same active ingredient

Angettes 75 Aspro clear Caprin (300mg)
Caprin (75mg) Disprin Disprin direct
Maximum strength aspro clear Nu-seals (300mg) Nu-seals (75mg)

Aspirin tablets, dispersible tablets, enteric-coated tablets and suppositories are also available without a brand name, ie as the generic medicine.