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The Gemora relates that Rabbah was found (by the king’s messenger) in Pumbedisa. The messenger was staying in the same inn as Rabbah. They (the workers at the inn) brought the messenger a plate of food and two cups two drink, and they then took away the plate. He turned his face away. [Rashi explains that the fact that he had two cups had given the demons reason to help damage him (in a life-threatening manner, as is apparent from the Gemora below). This is as the Gemora explains at the end of Pesachim, that an even number of drinks can cause this.] The workers asked Rabbah: What should we do? It is the king’s messenger! [They realized that this was the messenger of the king, and did not want to injure him.] Rabbah said: Bring him a plate and one drink, and take away the plate and he will be better. [He advised to give him one more drink, as this caused his drinks to be a total that was an odd number, giving the demons no more reason to harm him.] They did this, and this helped the situation.
The Tosfos HaRosh asks: Why should there be a danger here for drinking a pair of drinks? The Gemora in Pesachim (110b) states that there is no concern regarding a guest, for he does not know how many cups the host will offer him; it is therefore regarded as if he drank one and changed his mind and drank another. These two cups do not combine and there is nothing to worry about!?
He answers that the king’s messenger, on account of his prominence, is regarded as the host, since they will provide for him anything that he requests. It would therefore be a danger for him to have a pair of drinks.