A plausible explanation
Iron commonly darkens stool, so the patient assumed the change was harmless. There was also intermittent upper abdominal discomfort.
The clue in the story
The stools had become sticky and tar-like, with increasing weakness and light-headedness on standing. The pulse was faster than usual.
The twist
The patient had an upper gastrointestinal bleed rather than simple iron-related stool discoloration.
Take-home message
Medication side effects can mimic disease, but appearance, associated symptoms and haemodynamic clues matter. Tarry stool with dizziness, weakness or anaemia warrants urgent evaluation.
Diagnostic principle: do not let the first plausible label stop the search when an important detail does not fit.
Educational note: This is a teaching composite, not a substitute for patient-specific diagnosis or treatment. Real patients require direct clinical assessment and appropriate investigations.