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 over a prolonged time period is disclosed. In certain embodiments, the dose is intravenously administered in a time period from about 10 minutes to about 3 hours, preferably from about 10 minutes to about 30 minutes. In other embodiments, intravenous doses are administered at suitable time intervals over a time period from about 3 hours to 48 hours.