You probably have inherited a weakness in the walls of your leg veins. As the veins balloon out, blood flow slows and clots form, causing thrombophlebitis. Some diseases, such as leukemia and polycythemia, are marked by an over-abundance of blood cells. This predisposes an individual to clotting, because as viscosity increases, the blood flows less briskly and may coagulate inappropriately. These conditions can also be diagnosed by special testing.
Smoking causes increased platelet stickiness.
Therefore, it predisposes users to accelerated blood clotting. Furthermore, smoking reduces the level of oxygen in the blood stream, leading to increased production of red blood corpuscles, enhanced viscosity and stagnation. Some medications, such as birth control pills, add to this danger of smoking. In summary, your genetic inheritance, consumption of cigarettes and use of birth control pills are working together to increase the likelihood of inappropriate intravascular coagulation. Because you have no control over your predisposition to varicose veins, you will have to take measures to reduce the other risk factors.