Constipation actually gives rise to a process of self-poisoning. Thus, auto-intoxication is the process whereby the body literally poisons itself by maintaining a cesspool of decaying matter in its colon. During fasting,  the concentration of toxins expunged from the body and appearing in the blood can increase ten times above normal concentrations. The released toxins can either exacerbate the symptoms being treated or create their own symptoms such as headaches, body ache, joint pain, dizziness, sweating, general malaise, sore throat, nausea and/or flu-like  symptoms.

Notably also, constipation, impaction, and bowel obstruction are common problems for oncology patients. Further, when there is a weak area of the bowel that is sluggish and toxic, there is the possibility of these waste products being re absorbed back into the system, usually migrating back into the blood , then settling in the various weak tissue areas through out the body. 

Note: Even small doses of opiods, in combination with other factors like decreased fibre and inactivity, can lead to constipation. 

At the extreme end of the constipation  spectrum is fecal impaction, a condition in which stool hardens in the rectum and prevents the passage of anystool. In some cases, complete bowel obstruction may happen on even small doses of opiods which may result in the person vomiting feces. This can be minimized with proper attention to constipation early on. The person may need a stool softener as well as a laxative.

Further findings suggest that Constipation actually gives rise to a process of self-poisoning. Thus, auto-intoxication is the process whereby the body literally poisons itself by maintaining a cesspool of decaying matter in its colon. Compare  Stool Facts  with  Constipation Factoid.