Aspirin should not be taken by people who are allergic to ibuprofen or naproxen,^[81][82] or who have salicylate intolerance^[83][84] or a more generalized drug intolerance to NSAIDs, and caution should be exercised in those with asthma or NSAID-precipitated bronchospasm. Owing to its effect on the stomach lining, manufacturers recommend that people with peptic ulcers, mild diabetes, or gastritis seek medical advice before using aspirin.^[81][85] Even if none of these conditions are present, there is still an increased risk of stomach bleeding when aspirin is taken with alcohol or warfarin.^[81][82] Patients with hemophilia or other bleeding tendencies should not take aspirin or other salicylates.^[81][85] Aspirin is known to cause hemolytic anemia in people who have the genetic disease glucose-6-phosphate dehydrogenase deficiency (G6PD), particularly in large doses and depending on the severity of the disease.^[86][87] Use of aspirin during dengue fever is not recommended owing to increased bleeding tendency.^[88] People with kidney disease, hyperuricemia, or gout should not take aspirin because aspirin inhibits the kidneys' ability to excrete uric acid and thus may exacerbate these conditions. Aspirin should not be given to children or adolescents to control cold or influenza symptoms as this has been linked with Reye's syndrome.^[4]

For some people, aspirin does not have as strong an effect on platelets as for others, an effect known as aspirin resistance or insensitivity. One study has suggested that women are more likely to be resistant than men^[89] and a different, aggregate study of 2,930 patients found 28% to be resistant.^[90] A study in 100 Italian patients found that of the apparent 31% aspirin resistant subjects, only 5% were truly resistant, and the others were noncompliant.^[91]