This study will examine the effect of a class of antidepressant medications called selective serotonin reuptake inhibitors (SSRIs) on platelet function. Platelets are small blood cells that help stop bleeding after injury to a blood vessel by forming a clot, or plug, in the vessel. Some medications impair platelet function, leading to increased bruising and bleeding. SSRIs decrease an important platelet component called serotonin, which may cause bleeding in some patients. SSRIs include fluoxetine (Prozac), sertraline (Zoloft), paroxetine (Paxil), fluvoxamine (Luvox) and citalopram (Celexa).