Aspirin: doses, uses, side effects

Aspirin is an anti-inflammatory drug used to treat and prevent numerous medical conditions. A big one is inflammation. 

Inflammation is actually a defense mechanism against danger. If your immune system encounters an invader like bacteria, a virus, or toxins, your body responds by initiating inflammation. This stimulates your immune system to release chemicals that destroy the invader. Classic signs of inflammation include fever, warmth, and swelling or pain in injured tissue. 

While inflammation is necessary, it isn't always beneficial. Certain health conditions can cause the immune system to respond so vigorously that there ends up being too much inflammation. This can then backfire and damage tissues the immune system was trying to protect in the first place. Severe or chronic inflammation can also lead to conditions like arthritis, cancer, diabetes, digestive problems, and heart disease. 

Symptoms from inflammation, like fever and pain, can cause significant discomfort. Aspirin can be used to control or reduce this inflammation, helping to alleviate symptoms.  

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Aspirin, or acetylsalicylic acid, is a type of medication known as a non-steroidal anti-inflammatory (NSAID). NSAIDs work by blocking a chemical called cyclooxygenase (COX), which is involved in causing inflammation.  

Aspirin is also found under the brand names Ascriptin, Bufferin, Bayer, Durlaza, and Aspergum. 

Aspirin is approved by the U.S. Food and Drug Administration (FDA) for a variety of purposes. The most common uses of aspirin are:

• Fever reducer and pain reliever: Fever and pain are common symptoms of inflammation. Aspirin is often used to treat fever and relieve pain from afflictions like joint pain, headaches, and muscle pain.  

• Heart attacks: People who are treated with aspirin during a heart attack are less likely to die from it. They are also less likely to have a second heart attack if they continue to take daily preventive low dose aspirin (FDA, 2015). 

• Cardiovascular disease: Aspirin can prevent blood vessels from becoming blocked and forming potentially deadly blood clots. This is especially important for people with high cholesterol or those who have undergone specific types of heart surgery. 

• Stroke: Aspirin blocks the production of chemicals that cause our blood to clot. People who’ve had a stroke caused by a blood clot in their brain may be told to take aspirin to lower the risk of a second stroke. 

• Kawasaki disease:  Aspirin typically cannot be used in children due to the risk of a condition called Reye’s syndrome. Kawasaki disease is the only condition in kids for which aspirin can be used as a treatment. Kawasaki disease can damage arteries in the heart, and aspirin is used to prevent this. 

• Arthritis: Aspirin is used to treat pain and swelling in joint diseases like rheumatoid arthritis and osteoarthritis.

Aspirin can also be used off-label to treat other conditions. Off-label means taking a medication not explicitly approved by the FDA for that particular use. But under the right circumstances, as we’ll see below, a healthcare provider might prescribe aspirin off-label.

• Heart attacks: Some studies suggest that taking aspirin can lower the risk of heart attack, even in people with no history of heart disease (Abdelaziz, 2019). 

• Cancer: Aspirin might lower the risk of death from some cancers (like colon cancer) and decrease the risk of cancers in the digestive tract (Rothwell, 2011). However, some researchers disagree. In one clinical trial, adults taking daily aspirin actually had an increased incidence of cancer (McNeil, 2018).

• Preeclampsia: Preeclampsia is a condition in pregnant people characterized by dangerously high blood pressure that can lead to seizures and is dangerous for both mother and baby. Low doses of aspirin are sometimes used to prevent this condition (ACOG Committee, 2018).

Aspirin can make you produce more stomach acid, so many side effects are related to the digestive system. Common symptoms include stomach pain, nausea, heartburn, irritation, and mild gastrointestinal bleeding. 

Serious side effects of aspirin are less common and include: 

• Excessive bleeding: Since aspirin interferes with blood clotting, severe bleeding is a possible side effect. Bleeding occurs most commonly in the stomach and intestines, but it can also occur in organs like the brain. 

• Allergic reactions: Aspirin can cause an allergic reaction in some people. Signs of this include a severe rash, flushing, and chest tightness or difficulty breathing. 

• Asthma: Like other NSAIDs, aspirin can induce an asthma attack in susceptible people, making it difficult to breathe. 

• Liver damage: High doses of aspirin or prolonged use of this medication can damage the liver. Symptoms of liver damage include nausea, vomiting, abdominal pain, confusion, and bleeding. This is even more likely if you take certain other medicines or use substances that can impair liver function, such as acetaminophen or alcohol. It’s important not to have three or more alcoholic drinks a day while taking aspirin.  

• Reye syndrome: Reye syndrome is uncommon but very dangerous. It usually occurs in children taking aspirin while they have a virus––especially the flu or chickenpox. Scientists haven’t figured out why it happens, but Reye syndrome causes damage to cells that can lead to liver failure and brain damage. There is no cure for Reye syndrome. For that reason, children should not be given aspirin. 

• Kidney damage: Aspirin and other NSAIDs (like ibuprofen) can cause damage to the kidneys, especially when used together. 

• Tinnitus: High doses of aspirin can cause a buzzing or ringing in the ears known as tinnitus. Although there are treatments to reduce symptoms, it can be permanent.  

Aspirin is available in different dosages, depending on the brand name. It is available over-the-counter (OTC) without a prescription. 

Oral tablets come in doses of 81 mg, 325 mg, 500 mg, and 650 mg. The 81 mg dose is also available in a chewable form. For those unable to take aspirin by mouth, rectal aspirin or suppositories are available in 60 mg, 120 mg, 200 mg, 300 mg, and 600 mg doses.  

The starting dose of aspirin depends on what it’s being used for. For example, a person with fever or muscle aches usually starts with around 325 mg every 4–6 hours as needed. If someone is taking aspirin to prevent a second heart attack, they might begin with an 81 mg dose once a day. A healthcare provider can help you figure out which aspirin dosage is best for your needs. 

Aspirin can cause adverse reactions when combined with other drugs. Some examples of medications to avoid on aspirin include: 

• Anticoagulant medications (blood thinners): Blood thinners interfere with blood clotting and are used to prevent blood clots in people with a tendency to develop them. Since aspirin also interferes with blood clot formation, taking aspirin and blood thinners can increase your risk of dangerous and even life-threatening bleeding when taken together. Examples of anticoagulants include heparin and warfarin. 

• Non-steroidal anti-inflammatory drugs: Other NSAIDs, such as ibuprofen, can interfere with the benefits aspirin has on the heart. The combination can also increase the risk of kidney damage. 

• Immune suppressors: Aspirin and immune suppressors like methotrexate are both used to treat conditions like rheumatoid arthritis. However, aspirin can interfere with the kidneys’ ability to clear methotrexate from the body, resulting in an increased risk of toxic effects (Colebatch, 2011).   

People with certain risk factors or underlying conditions have an increased risk of developing adverse effects from aspirin. The following groups should consult a healthcare provider before taking aspirin (FDA, 2015): 

• People with a known allergy to aspirin

• People with a high risk for bleeding (like those with hemophilia)

• Individuals taking blood thinners

• Those with uncontrolled high blood pressure

• Children (especially those with viral infections) 

• People with a history of stomach or intestinal ulcers or bleeding

• People with a history of liver or kidney disease

Baby aspirin

Despite its name, baby aspirin is no longer meant for babies. It was used to treat fever and pain in children for many years––until researchers discovered its connection to Reye syndrome. However, the term baby aspirin persists. 

Baby aspirin, also called low-dose aspirin, refers to an 81 mg pill or chewable tab. This dosage is typically used for prevention rather than for treatment. Some of the things healthcare providers use baby aspirin to prevent include: 

• Heart attacks after a prior one

• Stroke after a prior one or transient ischemic attack (similar to a stroke)

• Blood clots and blood vessel damage following Kawasaki disease

Baby aspirin has often been used off-label to prevent heart attacks and strokes before an initial incident or to lower the risk of colorectal cancer. However, recent recommendations by a U.S. panel of experts proposed a change to this practice. They suggest that the risk of side effects outweighs the benefits of daily aspirin for this purpose. They advise providers not to recommend baby aspirin to prevent a first heart attack, first stroke, or cancer (U.S. Preventive Services Task Force, 2021). 

Patients with specific conditions may benefit from using baby aspirin as a preventive measure, but this isn't without risks. Even at low doses, continuous aspirin use increases the risk of bleeding in the digestive tract and brain (Sutcliffe, 2013). It’s always a good idea to speak to a healthcare professional before starting any new medication. 

Aspirin/dipyridamole (Aggrenox)

Because of its ability to interfere with blood clotting, it can be used to prevent blood clots in people at a higher risk of developing them. In some cases, aspirin is combined with another anticoagulant (blood thinner) called dipyridamole to further prevent clots. 

Dipyridamole changes chemicals in the body to open up or dilate blood vessels. This in turn interferes with platelet clumping and prevents clots (Ciacciarelli, 2015). People who’ve had a stroke due to a blood clot in the brain are at risk of having a second one. In this case, aspirin/dipyridamole may be prescribed to decrease the risk of clots and future strokes. 

Some clinical trials also show this combination is more effective at preventing further clots than aspirin alone (Greving, 2019). There is still a risk of bleeding, so a healthcare provider must monitor its use. 

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Current version

October 15, 2021

Written by

Gina Allegretti, MD

Fact checked by

Yael Cooperman, MD

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About the medical reviewer

Yael Cooperman, MD, Ro

Yael Cooperman is a physician and works as a Senior Manager, Medical Content & Education at Ro.