A method of treating a subject for a serotonergic neurotransmission dysregulation disorder, comprises administering the subject a serotonin enhancer (e.g., a serotonin reuptake inhibitor) in an amount effective to treat the disorder; and concurrently administering the subject 5-hydroxytryptophan in an amount effective to enhance the activity of the serotonin enhancer, (e.g., serotonin reuptake inhibitor). In preferred embodiments the disorder is depression, anxiety, or substance abuse.