We summarize the clinical evidence for each medical benefit here. We begin with our assessment of the strength of evidence within each category, followed by a brief summary of individual studies or reviews of several studies. In assessing the strength of evidence, we consider the study design, number of participants, and the size of the treatment effect (how much outcomes changed with treatment).

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In several very large studies, regular aspirin use reduced mortality among people with adenocarcinoma, especially those without metastasis at diagnosis. People with cancer as a whole who took low-dose aspirin during treatment with immune checkpoint inhibitors showed better progression-free survival and lower risk of metastasis.

In one study, people with stage 4 colorectal cancer and normal body mass index before diagnosis showed lower mortality when treated with aspirin, but not people with obesity.

People with bladder cancer treated with aspirin showed moderately better survival, with more benefits from taking aspirin at least 3 times a week, in a very large study.

People with breast cancer treated with aspirin showed lower cancer-specific mortality and lower risk of distant metastasis in many studies.

People with colorectal cancer treated with aspirin after diagnosis have shown better survival in many studies. Aspirin lowered risk of metastasis among people with colorectal cancer in many large studies. Benefits seem to be stronger with colon cancer than with rectal cancer.

People with liver cancer showed better survival when treated with aspirin in many studies. People with biliary tract cancers (gallbladder cancer, cholangiocarcinoma, and/or ampulla of Vater cancer) showed better survival in one large study. Aspirin use did not lead to better survival among people with esophageal or stomach cancer in combined analyses of studies.

People with head and neck cancer with certain gene changes (PIK3CA mutations or amplification) showed better survival when using NSAIDs—predominantly aspirin—after diagnosis in one study, but overall, aspirin use has not led to better survival among people with head and neck cancer in several studies.

People with inoperable non-small cell lung cancer treated with aspirin showed better overall survival in one study.

In a study of people with diffuse large B-cell lymphoma, aspirin use did not show an effect on survival or tumor response after chemoimmunotherapy (chemotherapy combined with immunotherapy).

Aspirin use did not show an effect on survival among people with ovarian cancer in many studies.

In many studies, aspirin use reduced distant metastasis among people with prostate cancer, but it did not affect cancer-specific mortality.

Aspirin use may lead to more immune activation against colorectal cancer tumors.

Aspirin is widely used as an anti-inflammatory agent.

In a study of people with rectal cancer, aspirin did not reduce bladder toxicity during neoadjuvant long-course radiation therapy.

In a study of people with rectal cancer, aspirin did not reduce rectal toxicity during neoadjuvant long-course radiation therapy.

People treated with aspirin experienced fewer vascular disease events in one very large study.

The use of codeine plus aspirin reduced chronic cancer pain in several studies, but side effects of long-term use—increased bleeding risk and ulceration of the stomach—outweigh the benefits.

Aspirin use does not show much effect on risk across cancer types as a whole, and its effect on the risk of death from cancer is uncertain.

Aspirin use lowered the risk of distant metastasis in large studies.

Aspirin use did not affect the risk of bladder cancer in many studies.

Aspirin use did not affect the risk of brain cancer in many studies.

In many studies, people with breast cancer as a whole showed lower risk of recurrence when treated with aspirin, but one large study found no effect on recurrence of one type of breast cancer (high-risk, HER2-negative breast cancer). Many studies have also found that people taking aspirin showed slightly lower risk of breast cancer, especially estrogen receptor-positive tumors.

Two studies found that people with colorectal cancer had a lower risk of recurrence when treated with aspirin. However, a combined analysis of studies did not show a reduced risk of advanced, abnormal growth of tissue (neoplasia) in those with a history of abnormal colorectal tissue growth. People without cancer using aspirin have shown lower risk of colorectal cancer in many studies.

Colorectal cancer and pancreatic cancer are listed separately.

People treated with aspirin showed moderately lower risk of recurrence of liver cancer in many studies. People without cancer using aspirin regularly showed a slightly lower risk of gastrointestinal tract cancers as a whole, with more benefit for people under age 65 and for some types of cancer:

• Moderately lower risk of adenocarcinoma of the esophagus and gastric cardia or of squamous-cell esophageal cancer
• Moderately lower risk of liver, bile duct, or gallbladder cancer
• Moderately lower risk of liver cancer among people with chronic liver diseases including viral hepatitis or cirrhosis
• Moderately lower risk of stomach cancer

People older than 65 showed little or no benefit on risk of esophageal or stomach cancer from using aspirin in a very large study.

Ovarian cancer is listed separately.

People using aspirin have shown slightly lower risk of endometrial cancer in many studies, with stronger benefits among obese women. Aspirin use did not affect risk of uterine cancer for people over age 65 in a very large study.

Studies have not shown a clear benefit of aspirin use in reducing the risk of head and neck cancer.

Aspirin use did not lower risk of kidney cancer in many studies.

People using aspirin show a slightly lower risk of chronic lymphocytic leukemia/small lymphocytic lymphoma but not a lower risk of leukemia as a whole across many studies.

Even after many studies, the link between aspirin use and the risk of lung cancer is unclear. While many studies show some benefit, that may depend on the type of lung cancer, the dose of aspirin, and the age or sex of the people treated with aspirin.

The benefits of using aspirin for reducing risk of lymphoma depend on the specific type of lymphoma:

• No evidence of an effect on risk of Hodgkin lymphoma across many studies
• Slightly lower risk of non-Hodgkin lymphoma only among women in a very large study
• Slightly to moderately lower risk of chronic lymphocytic leukemia/small lymphocytic lymphoma across many studies
• No evidence of an effect on follicular lymphoma or large B-cell lymphoma across many studies

Overall, not much evidence shows benefits from using aspirin on the risk of skin cancer. The benefits depend on the specific type of skin cancer:

• Slightly lower risk of basal cell carcinoma among people treated with aspirin, with more benefits among people with a history of skin cancers or a high prevalence of actinic keratoses in one study
• Insufficient or weak evidence of lower risk for other types of skin cancer across many studies.

People, including people with risk factors for ovarian cancer, treated with aspirin have shown slightly lower risk of ovarian cancer across many studies.

People treated with aspirin have shown moderately lower risk of pancreatic cancer across many studies, although older adults showed no benefit in one very large study.

African American men with prostate cancer treated with aspirin showed a moderately lower risk of biochemical recurrence (an increase in PSA after treatment) after radiation therapy when treated with aspirin. Aspirin use has led to slightly lower risk of prostate cancer, especially of advanced or lethal cancer in many studies.

Aspirin use had no effect on risk of thyroid cancer across several studies.

People treated with both aspirin and statins have shown substantially lower risk of gallbladder cancer in one large study and moderately lower risk of Barrett’s esophagus in one mid-sized study.