Venous thromboembolism (VTE) remains a great challenge because of its frequency and of its potential severity. However, VTE treatment can also lead to iatrogenic complications. We report a case of thigh haematoma by a 83-year-old woman under fondaparinux for a solear thrombosis. Then we discuss the indications of Unfractionated Heparin (UFH), Low-Molecular-Weight Heparins (LMWH) and Fondaparinux, which are the three classes of rapidly acting anticoagulant treatments nowadays available. As their efficacy is comparable, the choice between these classes relies on the risk of adverse effects, which depends on some patient's characteristics. LMWH and fondaparinux are contra-indicated by the patients with a renal clearance under 30 ml/min. Only UFH are authorized during the whole pregnancy even though LMWH are more and more used. Fondaparinux has proven its safety by patients over 100 kg. UFH requires a daily biological management whereas it is optional for LMWH and fondaparinux, as long as their contra-indications are taken into account. No Heparin-induced-thrombocytopenia Syndrome (HIT-Sd) has been proven yet under fondaparinux so that platelets management seems not necessary, contrary to UFH and LMWH which require a twice-weekly platelets count. The accuracy of the therapeutic indication should result in the best benefit/risk assessment.