According to a 2021 article in the Clinical Medicine Journal, NSAIDs like ibuprofen can cause stomach ulcers as well as damage, irritation, and swelling in the small intestine. This can lead to gastrointestinal bleeding and perforation. The study authors further note that bleeding can either be overt — causing dark, sticky poop due to the presence of blood — or hidden, detectable only by the resulting iron deficiency.
A 2013 article in Case Reports in Hematology further states that in very rare instances, ibuprofen can induce autoimmune hemolytic anemia (AIHA). With this condition, the body’s immune system mistakes its own red blood cells for foreign invaders and begins to attack them causing them to burst. As shown in this case study, it can lead to quite serious anemia within a matter of days. If people with AIHA do not stop taking the offending medication right away, it can potentially lead to death.
It should be noted, however, that this condition is estimated to only affect one person out of every million. Additionally, other drugs can also cause AIHA and it is thought that NSAIDs comprise less than 15% of all cases.