irritable bowel syndrome
"I was Prescribed this for neuralgia. However I noticed after a few weeks my irritable bowel syndrome was gone. GONE! When my Dr wanted to switch me to Gabapentin for the nerve pain. I insisted on continuing nortriptyline. I would rather have the nerve pain than irritable bowel syndrome. 
Word of caution- when I was increased to ~60mg day I became severely constipated with abdominal pain. This medicine can cause paralytic ileus so be careful. I take 10-20mg day and seem to be okay."