From one of the enema websites that I moderated:
'Everybody's body is different. In principle - and of course when done with proper care and knowledge - enemas as such are not dangerous. Having said that, there are some dangers. As always, you must be careful when you're interfering with any of the body's routines. Cautions exist for people with heart diseases, cara-related diseases (asthma and such), diabetics, people with epilepsy, any woman during or just after pregnancy and in general for people who are not feeling well. In this case all stomach, intestinal and metabolism related diseases should be added to this list as well of course. Also, enemas may have a devastating impact on people with an alcohol or drug addiction.
"Optimistic" quantities of fluid used and a high frequency of use may also cause all sorts of problems, the most important one being the fact that the bacteria in the intestines and the muscles around them may "get lazy" and the metabolism may become less effective. Keep your quantity and frequency reasonable.
Improper administration of an enema may cause electrolyte imbalance (with repeated enemas) or ruptures to the bowel or rectal tissues resulting in internal bleeding