Hi & Welcome. I appreciate your concern for your beloved husband. It can cause harmful effects if both the medications are taken together. Amitryptiline (tricyclic antidepressant) when taken while on Fluoxetine (selective serotonin reuptake inhibitor) may cause excessive sedation (drowsiness), dry mouth, blurring of the vision, constipation and urinary retention. It can also result in altered consciousness, confusion, poor muscle coordination, abdominal cramping, shivering, dilation of the pupils, sweating, high blood pressure, and increased heart beat. So, it is not advisable to take both the medications together. He should be clinically examined by an orthopedic surgeon and to detect any underlying cause of back pain like pinched nerve due to disc prolapse,etc by MRI and then needs to be treated accordingly. Hope this will help.