Historically, yohimbine was used as an aphrodisiac, and is still sometimes used in veterinary medicine (although now largely replaced by atipamezole) for reversing the effects of alpha-2 agonists such as medetomidine which are used as sedatives during surgery. 
α2-blockers have found medical uses in treating depression; the tetracyclic antidepressantsmianserin and mirtazapine are α2-blockers, although their efficacy as antidepressants may come from their activity at other receptor sites.
Mechanistically, α2-blockers significantly increase adrenergic, dopaminergic and serotonergic neurotransmitters, and induce insulin secretion and decreases blood sugar levels.
Withdrawal from α2-blockers can be difficult or dangerous as the global down regulation of neurotransmitters may cause symptoms of depression and other neurological problems, and increased blood sugar levels together with decreased insulin sensitivity can cause diabetes. Reduced microcirculation together with adrenaline supersensitivity in organs such as liver can also be difficult problem when withdrawing from adrenergic blockers.
^Lemke, KA (June 2004). "Perioperative use of selective alpha-2 agonists and antagonists in small animals.". The Canadian veterinary journal. La revue veterinaire canadienne45 (6): 475–80. PMID15283516.
^Haapalinna A, Leino T, Heinonen E (November 2003). "The alpha 2-adrenoceptor antagonist atipamezole potentiates anti-Parkinsonian effects and can reduce the adverse cardiovascular effects of dopaminergic drugs in rats". Naunyn Schmiedebergs Arch. Pharmacol.368 (5): 342–51. doi:10.1007/s00210-003-0827-z. PMID14566451.