Birth control pills can be very effective at preventing pregnancy. However, the hormones in some birth control pills may increase the risk of blood clots. Women using certain types of pills may also be putting themselves at risk for venous thrombosis, dangerous blood clots in the veins. "Currently available oral contraceptives still have a major impact on thrombosis occurrence and many women do not use the safest brands with regard to risk of venous thrombosis," explains BBC News. Birth control pills using the combination of hormones oestrogen and progestogen seem to carry the highest risk. Other hormones noted were desogestrel, gestodene or drospirenone. Blood clots while on the birth control pill are rare. If a patient is very concerned about venous thrombosis, she should discontinue use and consult a physician.