The refusal to eat is often both physical (the build up of toxins) and psychological (the association of food with feeling sick). The answer to this is to provide
anti-nausea
medication, which will help encourage a dog to start eating again - and in turn, start having enough energy to enjoy life a little more.
But, there are so many different anti-nausea medicines, which makes it very confusing to know which one is the best in treating this symptom.