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 certain embodiments, the dose of tramadol is from about 45 mg to about 80 mg and the second (and optionally) third doses are intravenously administered at intervals of from about 2 to about 3 hours, and thereafter the tramadol is intravenously administered at a dosing interval of about 4 to about 6 hours, until the patient no longer requires treatment with tramadol. 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).