Smoking thickens the blood by increasing the production of red blood cells. This promotes clotting and can close off a narrowed section of an important blood vessel.

Smoking stimulates the bone marrow to produce more red blood cells (polycythemia) as a response to carbon monoxide in cigarette smoke, which reduces oxygen-carrying capacity. This increased red blood cell count makes the blood more viscous (thicker). The thickened blood flows more slowly and is more prone to clotting. Combined with smoking-induced damage to blood vessel walls, this significantly increases the risk of clots that can block narrowed arteries, causing heart attacks or strokes. Therefore, True is correct.