It is forbidden for a husband to drink the leftover beverage from a wife’s cup while she is a Nidda. For example, if the wife was drinking soda and did not finish all the soda in the cup, the husband is not allowed to drink the remaining soda from her cup. The Sages enacted this measure as one of several safeguards against intimacy.
There are, however, a number of ways in which it would be permissible for the husband to finish the wife’s drink. Firstly, if the leftover soda was transferred to another cup or glass, the husband is allowed to drink the soda. Furthermore, he may drink the leftovers even from the original cup if some more soda was added. He may also drink the remaining soda if somebody else had drunk some of the wife’s leftovers beforehand. In all these situations, the husband may drink the leftovers because there is a "Heker" – a discernible reminder that they may not be intimate.
According to the strict Halacha, if a wife who is a Nidda drinks from a cup and does not leave over any beverage, the husband may use the cup immediately afterward. There is, however, a custom not to use a cup used by a wife (when she is a Nidda) before it is washed. Hacham Ovadia Yosef refers to this custom as a "Minhag Kasher" ("proper custom") which should be followed. Therefore, when one’s wife is a Nidda, he should preferably not drink from a cup from which she had drunk, until it is washed.
Summary: A husband may not drink a wife’s leftovers from her cup while she is a Nidda unless they are transferred to a different cup, some more beverage was added into the original cup, or somebody had drunk some of her leftovers before the husband. If the wife finished the drink, it is proper for the husband not to use her cup until it is washed.
