If one used his cup for Mayim Achronim, may he drink the remaining liquid after Benching?

One may drink the water left in his cup that was used for Mayim Acharonim. Nonetheless, some people are particular about this.

This refers only to the cup from which the water was poured. However one should not drink the leftover water in the cup into which the water was poured and many are particular not to use at all unless rinsed very well.

Sources: