סימן קנג סעיף ז

לֹא יִשְׁתֶּה מִשִׁיוּרֵי הַכּוֹס שֶׁשָׁתְתָה הִיא. וְאִם הִפְסִיק אָדָם אַחֵר בֵּינֵיהֶם, אוֹ שֶׁהוּרַק אֶל כּוֹס אַחֵר, שָׁרֵי. וְאִם שָׁתְתָה וְהוּא אֵינוֹ יוֹדֵעַ, וְרוֹצֶה לִשְׁתּוֹת מִכּוֹס זֶה, אֵינָהּ צְרִיכָה לְהַגִּיד לוֹ שֶׁשָׁתְתָה הִיא מִמֶּנוּ. (אֲבָל אִם יָדַע שֶׁשָׁתְתָה הִיא, אֶלָּא שֶׁלֹּא יָדַע שֶׁהִיא נִדָּה, כָּתַבְתִּי בְּסִפְרִי לֶחֶם וְשִׂמְלָה, כִּי נִרְאֶה דִּצְרִיכָה לְהַגִּיד לוֹ). הִיא מֻתֶּרֶת לִשְׁתּוֹת מִשִּׁיוּרֵי כּוֹס שֶלּוֹ. יֵשׁ אוֹמְרִים, דִּכְשֵׁם שֶׁאָסוּר לִשְׁתּוֹת מִשִׁיּוּרֵי כּוֹס שֶׁלָּהּ, כָּךְ אָסוּר לֶאֱכוֹל מִשִׁיּוּרֵי מַאֲכָל שֶׁלָּהּ.

You are not allowed to drink what she leaves over in her cup. But if someone else drank from the cup after her, or it has been poured into another cup you are permitted to drink it. If she drank from it and you are unaware of it, and you want to drink from that cup, she need not tell you that she drank from it. (But if you know she drank from it but do not know that she is a niddah, it would seem that she has to inform you). She is permitted to drink the leftovers from your cup. Some authorities maintain that just as you are forbidden to drink the leftovers from her cup so too you are forbidden¹¹All the conditions that make it possible for you to drink her leftovers make it possible for you to eat her leftovers. to eat from her leftover food.¹²Only if it is one piece of food. But if there are two pieces on a plate and she eats one, the second one is not considered her leftover (Igros Moshe Resp. 92). If it is a food made up of many small pieces (like cut-up salads etc.) the whole salad is like one piece and is considered her leftover.

תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)