Some side effects can be severe, or a sign of a serious problem if they occur. One such side effect would be gastrointestinal bleeding, which can especially occur if the aspirin is combined with nonsteroidal anti-inflammatory drugs, or NSAIDs. Aspirin can sometimes cause allergic reactions as well, manifesting themselves in rashes, hives, a tightness of the chest, or many other reactions. Aspirin can ironically cause severe stomach pains, as well as vomiting, diarrhea, bloody stools, or plenty of other unpleasant effects. Some people might even experience ringing in their ears or some amount of hearing loss. Sometimes aspirin can even cause changes to your mood or mental state, and it is possible to have hallucinations when taking aspirin, too. If you have these or any other severe effects, you should immediately seek medical help.
Other side effects can be much more minor. Again, somewhat ironically, aspirin can sometimes actually cause nausea, headaches or lightheadedness. There can also be minor variations on some of the side effects listed above, such as diarrhea, or others such as dizziness that can occur. Heartburn is another common side effect that many may experience.
Aspirin suppositories are available, and some people will experience rectal irritation as a minor side effect, along with some of the more severe effects listed above. They can also cause, as severe effects, rectal bleeding and Reye’s syndrome, which will manifest itself nausea, vomiting, and fever.
This is not a complete list of the symptoms that can occur. If you experience anything unusual, you should always call a doctor and seek their professional advice.