Hello, My mother in law is flying to South Africa which is approx. 11hrs flying time but a total of 16hrs travel time. She has no past history of DVT and does not normally take any anti coagulant therapy . How ever she does have incompetant valves in her deep veins ..her Gp has prescribed Clexane 20mg/0.2ml inj ..to be injected pre flight only ...and another injection for her to give pre homeward flight. My question is ..Knowing generally Clexane last s about 12 hrs ..is the one injection enough?