If constipation isn't treated it can cause complications, including:faecal impaction-this is when hard faeces collects in your rectum and you reach a point where you probably won't be able to have a bowel movement without treatment; overflow incontinence-this is when small amounts of loose faeces leak out around the impacted faeces, without you feeling it; bleeding from the anus; psychological problems, especially in children, eg holding in faeces because passing them has been painful in the past; development of haemorrhoids(piles).
Attribution: BUPA