Ask your vet or local club to be sure. If your rat is so ill he's refusing to eat or drink, you may be forced to use injections to medicate and hydrate him. The bacteria which helps both rats and people digest food can be killed off by antibiotics. When they are prescribed for your rat, ask if you can give him yogurt while he's on them. If the gut flora gets wiped out by the antibiotic your rat could develop diarrhea which can quickly lead to dehydration.
Dehydration is very serious for a rat.
Pedialyte, for human babies, is good for rats with diarrhea or dehydration. An injection of lactated ringers solution can help immensely when a rat is dehydrated. Any rat with diarrhea should be taken to the vet as soon as possible. When a rat is really sick he may not feel like eating, and can lose weight very quickly. This is not good.