A method of treating pain, e.g., acute post-operative pain, by administering to a human patient(s) a therapeutically effective dose of tramadol intravenously in a dosing regimen which includes one or more loading doses administered at shortened intervals as compared to dosing at steady-state is disclosed. In preferred embodiments, the intravenous dosing regimen provides a Cmax and AUC of tramadol is similar to the Cmax and AUC of an oral dose of 100 mg tramadol HCl given every 6 hours. In certain preferred embodiments, the dosing regimen comprises 50 mg IV tramadol at Hour 0, followed by 50 mg at Hour 2, 50 mg at hour 4, and 50 mg every 4 hours thereafter (e.g., until the patient no longer requires treatment with tramadol).