The past couple of times I applied to myself, I purged too soon, before all the bile and toxins had been released so I continued to vomit despite there being little or no liquid in my stomach which was unpleasant. In cases like that, you can just drink more water. Some people say that purging isn't necessary. That doesn't make any sense to me, I feel terrible before purging which would indicate that its the toxins released by the kambo that causes me to feel bad.
As for fasting, its necessary for the healing effects in my experience. Not just before, but I've come to the realisation that you need to fast afterwards, otherwise the symptom relief seems to come to an end. Namely, I've experienced lethargy coming back when I ate food within an hour after recovering from a kambo ceremony, whereas if I don't eat, I will be free of lethargy for the rest of the day. I see it as allowing the body to dedicate all its faculties to the healing process triggered by the kambo, whereas eating forces the body to dedicate some of these faculties to the digestion and metabolism of the food. Maybe its only certain types of food that should be avoided after the ceremony, such as foods that are difficult to digest or metabolise.