Doctor's view/Dr Muiris Houston:The risk of deep venous thrombosis (DVT) for a woman taking a combined oral contraceptive (commonly referred to as "the pill") is well established.
Along with other oestrogen-containing medication such as hormone replacement therapy (HRT), oral contraceptives carry a small increased risk of developing a DVT.
DVT is a form of venous thromboembolism which also includes a condition called pulmonary embolus (PE).
DVT refers to the formation of a blood clot in the deep-lying veins of the leg; PE occurs when a piece of this clot breaks off and travels from the leg, through the heart, and lodges in the lung.
A large clot can block one or more of the pulmonary arteries so that blood cannot travel from the right side of the heart to the lungs, rapidly causing death.
Thromboembolism is precipitated by three distinct processes: pooling of blood in the veins such as may occur with long-distance air travel; damage to the wall of the vessel; and the presence of any substance that increases the tendency for blood to coagulate, such as oestrogen.
The increased risk of DVT/PE associated with oral contraceptive use is low in absolute terms. This additional risk generally manifests itself by the fourth month of pill use and disappears about three months after stopping a combined contraceptive pill (i.e. one containing both oestrogen and progestogen).
Length of pill use does not alter the risk of clotting.
The absolute risk of developing thromboembolism in a healthy non-pregnant woman who is not taking the pill is about five in 100,000. The relative risk for someone taking the pill rises threefold, with an absolute risk of 15 in 100,000.
By comparison, the absolute risk of a DVT or PE in pregnancy is 60 in 100,000.
Other risk factors include heavy smoking and obesity. However, as the coroner noted, Julie Hennessy (31) did not have any of these risks, nor had she engaged in long-distance travel prior to her premature death.
The symptoms experienced by Ms Hennessy - calf pain and breathlessness - are associated with the presence of a clot in the leg and lung.
Women who are taking the combined contraceptive pill who notice leg pain, swelling or redness of the calf, or chest pain or breathlessness, should be assessed medically to check for evidence of a DVT or PE.