Combined oral contraceptive pills increase your risk of deep vein thrombosis (DVT). Prolonged immobility can also increase your risk, as can living at high altitudes. When flying, avoid alcohol and sleeping tablets, keep well hydrated, and exercise your legs regularly. Consider wearing properly fitted compression stockings. Consider alternate forms of contraception if you have other risk factors for DVT, such as smoking, obesity, varicose veins, or aged over 35.
High altitude travel can also raise the risk of DVT for women on COC. Consider alternative contraception if you are planning to stay at over 3500 metres for several weeks. The combined pill is definitely not recommended if you are spending over a week at very high altitude (4500 metres or higher).
Unexplained pain or swelling in either calf muscle is a warning of DVT. Stop taking your contraceptive pills immediately and seek medical advice.
Aspirin has not been shown to be effective at preventing deep vein thrombosis. It is associated with an increased risk of gastrointestinal bleeding and should not be used to prevent the condition.