Clexane is a medicine that is used to treat blood clots, or stop clots from forming inside the body.Clexane is commonly used to prevent clotting after an operation such as a hip replacement, or in certain heart conditions.ction. Clexane works by preventing certain molecules in the blood called clotting factors from working. Clotting factors are needed to from a clot (eg. on a cut or scratch). Sometimes, however, blood clots can form inside the body and travel to areas such as the lungs or brain, where they can from a block.