A person who is likely to be very toxic needs to prepare before beginning an extended water only fast unless they will be supervised by a fasting expert. Paul Bragg suggests starting slowly with one day a week fasts, then three days a month to allow the body to detox at a slower and safer rate. Personally, I believe fasting is easier if one takes the time to make sure the organs of elimination are in good working order. This includes the liver, colon and kidneys. A few weeks should also be taken to wean off certain "addictive" foods including caffeine, white Sugar and all artificial sweeteners.
It can be difficult to take the time to prepare for a water fast, but it is well worth it. So often people are simply too toxic to successfully fast and they feel like failures because they cannot continue beyond a day or two. If people would take time to prepare for their fast by starting slowly, weaning off certain foods, and cleansing their bodies then they are much more likely to be able to continue their fast and achieve their desired results.