Patients who developed thromboembolic complications when they received anticoagulant therapy were compared with control patients who did not develop the same symptoms, but were matched for other epidemiologic variables. The controls had more days of heparin sodium therapy and received higher doses of the drug. They also received anticoagulant treatment orally for more days while their prothrombin rates were below 30% and 25%, but these differences were not statistically significant. These data in combination with other studies, which show that anticoagulants administered orally probably reach their maximum antithrombotic effectiveness after the first week of administration, support the recommendation that every patient with thromboembolism should receive heparin therapy concurrently with anticoagulant therapy, administered orally, for at least seven to ten days.
Coon WW, Willis PW. Thromboembolic Complications During Anticoagulant Therapy. Arch Surg. 1972;105(2):209-212. doi:10.1001/archsurg.1972.04180080063011