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Aspirin: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on Oct 12, 2022.

1. How it works

  • Aspirin blocks the effects of cyclooxygenase-1 (COX-1) and COX-2 enzymes which prevents the synthesis of prostaglandins. Prostaglandins are hormone-like substances that modulate inflammation and are also involved in smooth muscle contraction and relaxation, blood vessel narrowing and widening, and blood pressure control.
  • Aspirin affects platelets because it inhibits the formation of thromboxane A2, a prostaglandin derivative. This reduces the ability of the blood to clot.
  • Aspirin belongs to the class of medicines known as nonsteroidal anti-inflammatory drugs (NSAID). It may also be grouped with nonopioid analgesics and antiplatelet agents.
  • Aspirin is a salicylate and is also known as acetylsalicylic acid.

2. Upsides

  • Used for the short-term relief of symptoms such as a headache, pain, and fever that occur as a result of colds, muscle trauma, menstruation, and toothache.
  • May be used to relieve signs and symptoms of Rheumatoid arthritis and other types of arthritis; systemic lupus erythematosus, and spondyloarthropathies.
  • Used in small doses following various heart-related revascularization procedures (such as coronary artery bypass graft [CABG]) to improve the flow of blood through the revascularized area.
  • Used to reduce the risk of death or another stroke in people with a history of stroke due to blood clots or previous stroke-like events.
  • Used to help prevent another heart attack or reduce the risk of death in people at risk of a heart attack, or who already have angina.
  • Used off-label for several other conditions such as preeclampsia, the emergency treatment of stroke or a heart attack, and to prevent blood clots in people with atrial fibrillation (who are unable to take anticoagulants). Off-label means it is not FDA approved for this use, but there may be data to show it is safe and effective.
  • Research has also shown that aspirin may help prevent colorectal cancer and the development of colorectal adenomas in people previously diagnosed with colorectal cancer. Aspirin may also reduce the incidence of colorectal cancer among people testing positive for hereditary nonpolyposis colon cancer and in carriers of Lynch syndrome.
  • Generic aspirin is available.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • Stomach pain/irritation, heartburn, nausea, and tinnitus (ringing in the ears). Cuts and minor wounds may bleed for longer than usual. Most side effects are rare at low dosages. The risk of side effects is increased with higher dosages, individual sensitivity, or with certain concomitant medications.
  • Rarely, stomach bleeding or hemorrhage from any site (the risk is higher in people who smoke, who drink more than 3 glasses of alcohol per day, or who take other medicines that affect bleeding time).
  • Not suitable for people with an allergy to NSAIDs; those with the triad of asthma, rhinitis, and nasal polyps; children and teenagers with a suspected viral illness; and in those with severe kidney or liver disease.
  • May cause an allergic reaction in people sensitive to other types of salicylates or tartrazine dyes.
  • Extended-release capsules of aspirin are available; however, these should not be used in situations where a rapid onset of action is required (such as immediately following a heart attack).
  • May interact with several different drugs including other anticoagulants or antiplatelet agents, carbonic anhydrase inhibitors, corticosteroids, other NSAIDs, SSRI antidepressants, and thrombolytics. Several herbs, vitamins, and other types of supplements may also interact. May cause false-negative or false-positive results on some diagnostic tests. Alcohol may increase the risk of bleeding.
  • Dosages of aspirin of more than 100 mg/day should be avoided at week 20 of pregnancy and later may cause fetal kidney problems leading to oligohydramnios (low amniotic fluid volume) and in some cases kidney impairment, or premature closure of the fetal ductus arteriosus if used during the last three months of pregnancy. Dosages of 100 mg/day or less should be used with caution. If aspirin at dosages such as 81 mg/day is given to women at high risk of preeclampsia, it should be initiated between 12 and 28 weeks gestation (optimally before 16 weeks) and continued until delivery; the use of aspirin at 81 mg/day dose for certain pregnancy-related conditions at any point in pregnancy is an exception to the FDA recommendations to avoid the use of NSAIDs in pregnancy at 20 weeks or later. Aspirin is excreted into breast milk in low amounts. Low-dose aspirin (75 to 162 mg/day) is considered compatible with breastfeeding. Large doses may result in rashes, platelet abnormalities, and bleeding in nursing infants. The risk for Reye's syndrome in infants with viral infections is unknown. The benefit should outweigh the risk.

Note: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. View complete list of side effects

4. Bottom Line

Aspirin relieves pain and inflammation and helps to thin the blood. It is used to treat and prevent a wide range of different conditions. Higher dosages are associated with stomach irritation and increased bleeding risk.

5. Tips

  • For a suspected heart attack, chew a non-coated 325 mg tablet of aspirin as soon as possible and call 911.
  • Unless you suspect you are having a heart attack, always seek your doctor's advice before taking aspirin.
  • Take aspirin with food or after meals. Try not to let yourself become dehydrated while taking aspirin.
  • Aspirin comes in various strengths and dosages vary depending on the condition being treated or prevented. Take aspirin as directed by your doctor. Note that this dosage may differ from that taken by family and friends, but the actual dosage depends on your condition and age.
  • For most heart conditions, the typical maintenance dose is 81 mg once daily.
  • Dosages given for pain relief are generally higher than those given for heart conditions.
  • Administer each dose of aspirin with a full glass of water, unless you have been told to restrict fluids.
  • Do not crush or chew delayed- or slow-release preparations.
  • Do not give aspirin to children or teenagers with flu symptoms, fever, chickenpox, or any suspected viral illness due to the possibility of Reye's syndrome (a rare but serious condition that causes swelling of the brain and liver).
  • Small amounts of antacids taken with aspirin may decrease stomach irritation.
  • More than three glasses of alcohol per day may enhance bleeding risk and gastrointestinal side effects.
  • Avoid buffered aspirin products if you are on a sodium-restricted diet.
  • Seek emergency help if you experience any head or neck swelling, difficulty breathing, or severe itching after taking aspirin.
  • Tell your doctor if you experience ringing in your ears, persistent stomach pain, persistent indigestion, or blackened stools while taking aspirin.
  • Treatment with aspirin may need to be stopped a couple of weeks before surgery; check with your doctor.
  • Do not use for at least 7 days after tonsillectomy or oral surgery unless directed by your doctor.
  • Tell your doctor if you are pregnant, intending to become pregnant, or breastfeeding because aspirin may not be suitable for you.

6. Response and effectiveness

  • Takes from 5 to 30 minutes (depending on formulation) for aspirin to affect platelet function. Chewed, non-coated formulations work faster.
  • Needs to be taken daily to inhibit new platelets that are constantly being released into the circulation; however, platelet inhibition lasts the platelet lifetime (~10 days) due to permanent inhibition of platelet COX-1 enzyme.

7. Interactions

Medicines that interact with aspirin may either decrease its effect, affect how long it works for, increase side effects, or have less of an effect when taken with aspirin. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with aspirin include:

  • ACE inhibitors such as benazepril, captopril, and enalapril
  • acetazolamide
  • anticonvulsants such as phenytoin or valproic acid
  • apixaban
  • beta-blockers such as atenolol, labetalol, or metoprolol
  • corticosteroids, such as prednisone and methylprednisone
  • dabigatran
  • digoxin
  • furosemide or other diuretics
  • lithium
  • medications for diabetes
  • medications for gout, such as probenecid
  • methotrexate
  • mifepristone
  • NSAIDs, such as ibuprofen, diclofenac, etodolac, and naproxen
  • tetracyclines, such as doxycycline and minocycline
  • tinzaparin
  • warfarin.

In general, any medicine that can increase the risk of bleeding (such as clopidogrel, SSRI antidepressants [eg, citalopram, duloxetine, fluoxetine, venlafaxine], fish oils) may interact with aspirin.

Note that this list is not all-inclusive and includes only common medications that may interact with aspirin, You should refer to the prescribing information for aspirin for a complete list of interactions.


Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use aspirin only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Copyright 1996-2023 Revision date: October 12, 2022.