The poisonous nature of carbon monoxide (CO) is, due to its ability to form a complex with haemoglobin which is about 300 times more stable than the oxygen haemoglobin complex. This prevents haemoglobin (in the red blood corpuscles) from carrying oxygen in the body and ultimately result¬ing in death.
Haemoglobin + CO ------> Carboxyhaemoglobin.