In large quantities, the effect of COHb is death - known medically as carboxyhemoglobinemia or carbon monoxide poisoning. However in smaller quantities COHb leads to oxygen deprivation of the body causing tiredness, dizziness and unconsciousness.
Hemoglobin bonds to carbon monoxide preferentially (200:1 more so) compared to bonding to oxygen, so effectively, COHb will not release the carbon monoxide, and therefore hemoglobin will not be available to transport oxygen from the lungs to the rest of the body. However, animals, such as a human, should survive with very small amounts of COHb in their blood with very little or no observable effects.
Pregnant smokers may give birth to babies of a lower birth mass as fetal hemoglobin takes up carbon monoxide more readily than in an adult, therefore the fetus of a smoker will suffer from mild hypoxia potentially retarding its development.