AIDS: Recent studies have suggested that serotonin reuptake inhibitors (SSRI's), prescribed for the relief of depression, can cause sexual dysfunction in up to fifty percent of those taking them. The SSRI's--including fluoxetine (Prozac), sertraline (Zoloft), and paroxetine (Paxil)--affect mood stabilization by promoting the transmission of the neurotransmitter serotonin, although enhancing serotonergic function can decrease libido or lead to erectile difficulties. As an alternative to lowering antidepressant dosages and risking losing therapeutic gains, administering serotonin-blockers, such as cyproheptadine (Periactin) and yohimbine (Yocon), has been shown to restore sexual function. However, the serotonin antagonist, cyproheptadine, causes sedation and can reverse the antidepressant or anti-obsessive effect of the SSRI. Yohimbine enhances transmission of the neurotransmitter epinephrine, increasing the flow of blood to erectile tissue and stimulating sexual desire by activating the cerebral cortex. Its drawbacks are increased levels of panic attacks and higher required dosages. Other potential biochemical stratagems are: amantadine (Symmetrel), bromcriptine (Parlodel), and buspirone (Buspar), which enhance dopamine and serotonin transmission; and bethanecol (Urechline), which enhances choline transmission. One study indicates improved sexual response when the nonserotonergic, mildly dopamine-enhancing buproprion (Welbutrin) is substituted for fluoxetine.