The primary aim is to determine the relative efficacy of exposure-based CBT for social phobia when conducted with adjunctive acute (prior to four of five sessions) administration of either Yohimbine hydrochloride (10.8 mg) or placebo during core exposure sessions. Based on the available evidence, the investigators hypothesize that acute treatment with Yohimbine hydrochloride prior to exposure-based CBT would facilitate the extinction of fear that occurs with this treatment and would enhance treatment outcome.