The clinical signs of carbon monoxide (CO) poisoning are highly variable and may change over time.

In cases with few symptoms, if the circumstances do not suggest CO poisoning, diagnosis can be difficult. As a result, many cases of CO poisoning go undiagnosed. The earliest and most common symptoms are headaches, fatigue, nausea, and vomiting. These signs can sometimes lead to an incorrect diagnosis of other conditions (recent or recurrent food poisoning without diarrhea, flu-like syndrome without fever).

In the most severe cases, symptoms include loss of consciousness, neurological or cardiovascular disorders that can lead to a coma, and ultimately death.

Food poisoning often affects groups of people, with symptoms varying from person to person depending on individual sensitivity and specific circumstances (such as the elderly, children, pregnant women, and people with pre-existing conditions like anemia or hemoglobinopathies, etc.).