Aspirin is a commonly used medication for pain relief, fever reduction, and blood thinning. However, like all medications, it can cause side effects. The most common side effects of aspirin are gastrointestinal symptoms, such as stomach pain, nausea, and heartburn [2]. Other side effects include dizziness, ringing in the ears, and allergic reactions such as hives or difficulty breathing [3]. Aspirin can also increase the risk of bleeding, especially if taken in high doses or for a long time [2]. Some people may also experience more serious side effects such as stomach ulcers, bleeding in the brain, or kidney problems [1]. It is important to speak with a healthcare provider before taking aspirin, especially if you have a history of stomach ulcers, bleeding disorders, or allergies to aspirin or other nonsteroidal anti-inflammatory drugs (NSAIDs) [2]. If you experience any severe side effects while taking aspirin, seek medical attention immediately [3].

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/ASPIRIN
[2] https://www.nhsinform.scot/tests-and-treatments/medicines-and-medical-aids/types-of-medicine/aspirin
[3] https://www.drugs.com/aspirin.html