The FDA says development of a potentially life-threatening serotonin syndrome may occur in treatment with four selective serotonin reuptake inhibitors: Lilly’s Cymbalta (duloxetine HCl) delayed release capsules, Wyeth’s Effexor (venlafaxine HCL) tablets and Effexor extended release capsules, Lilly’s Prozac (fluoxetine capsules and oral solution) and Lilly’s Symbyax (olanzapine and fluoxetine HCL capsules).