 Some antidepressants function partly as selective alpha @-@ 2 blockers , but the best @-@ known drug in that class is <unk> , which is extracted from the bark of the African <unk> tree . <unk> acts as a male potency enhancer , but its usefulness for that purpose is limited by serious side @-@ effects including anxiety and insomnia . <unk> can cause a dangerous increase in blood pressure . <unk> is banned in many countries , but in the United States , because it is extracted from a plant rather than chemically synthesized , it is sold over the counter as a nutritional supplement . 
