Siman 618
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א חוֹלֶה שֶׁצָּרִיךְ לֶאֱכֹל, אִם יֵשׁ שָׁם רוֹפֵא בָּקִי אֲפִלּוּ הוּא עוֹבֵד כּוֹכָבִים שֶׁאוֹמֵר: אִם לֹא יַאֲכִילוּ אוֹתוֹ אֶפְשָׁר שֶׁיִּכְבַּד עָלָיו הַחֹלִי וְיִסְתַּכֵּן, מַאֲכִילִין אוֹתוֹ עַל פִּיו, וְאֵין צָרִיךְ לוֹמַר שֶׁמָּא יָמוּת. אֲפִלּוּ אִם הַחוֹלֶה אוֹמֵר: אֵינוֹ צָרִיךְ, שׁוֹמְעִים לָרוֹפֵא; וְאִם הַחוֹלֶה אוֹמֵר: צָרִיךְ אֲנִי, אֲפִלּוּ מֵאָה רוֹפְאִים אוֹמְרִים: אֵינוֹ צָרִיךְ, שׁוֹמְעִים לַחוֹלֶה.
1 Laws of a Sick Person on Yom Kippur and in it is 10 paragraphs A Choleh (sick person) that needs to eat: if there is an expert doctor, even who is a Gentile, that says if he (the sick person) does not eat it is possible his sickness will worsen and become dangerous, we feed him on his word (the doctor). It is unnecessary (i.e obvious) to say (that he can eat, if the doctor says) maybe he'll die even if the sick person says it is not necessary. We listen to the doctor. Yet, if the sick person says I need to eat, even if 100 doctors say it is unnecessary, we listen to the sick person.
ב רוֹפֵא אֶחָד אוֹמֵר: צָרִיךְ, וְרוֹפֵא אֶחָד אוֹמֵר: אֵינוֹ צָרִיךְ, מַאֲכִילִים אוֹתוֹ. הַגָּה: וְהוּא הַדִּין לִשְׁנַיִם נֶגֶד שְׁנַיִם, וַאֲפִלּוּ קְצָתָן יוֹתֵר בְּקִיאִין מִקְּצָתָן כֵּן נִרְאֶה לִי.
2 One doctor says it is necessary (to eat), and one doctor says it is not necessary. We feed him. Rema: And similarly two against two, and even if one pair has more expertise than the other pair, so seems to me.
ג וְאִם הַחוֹלֶה וְרוֹפֵא אֶחָד עִמּוֹ אוֹמְרִים שֶׁאֵינוֹ צָרִיךְ, וְרוֹפֵא אֶחָד אוֹמֵר: צָרִיךְ; אוֹ שֶׁהַחוֹלֶה אֵינוֹ אוֹמֵר כְּלוּם וְרוֹפֵא אֶחָד אוֹמֵר: צָרִיךְ, וּשְׁנַיִם אוֹמְרִים: אֵינוֹ צָרִיךְ, אֵין מַאֲכִילִין אוֹתוֹ.
3 If the sick person and one doctor say it is not necessary (to eat). And (another) doctor says it is necessary. Alternatively, the sick person does not say anything and one doctor says it is necessary, and two say it is not necessary. We do not feed him (i.e. the sick person in either scenario.)
ד אִם שְׁנַיִם אוֹמְרִים: צָרִיךְ, אֲפִלּוּ מֵאָה אוֹמְרִים: אֵינוֹ צָרִיךְ, וַאֲפִלּוּ הַחוֹלֶה אוֹמֵר עִמָּהֶם שֶׁאֵינוֹ צָרִיךְ, מַאֲכִילִים אוֹתוֹ מֵאַחַר שֶׁשְּׁנַיִם אוֹמְרִים: צָרִיךְ. הַגָּה: וְהוּא הַדִּין אִם הַחוֹלֶה וְרוֹפֵא אֶחָד עִמּוֹ אוֹמְרִים: צָרִיךְ, אַף עַל פִּי שֶׁמֵּאָה רוֹפְאִים אוֹמְרִים: אֵינוֹ צָרִיךְ, מַאֲכִילִין אוֹתוֹ (טוּר), וְלֹא חַיְישִׁינָן דְּהַחוֹלֶה אוֹמֵר: צָרִיךְ מִשּׁוּם דְּמַאֲמִין לְרוֹפֵא זֶה שֶׁאוֹמֵר: צָרִיךְ (בֵּית יוֹסֵף בְּשֵׁם מהרי"א).
4 If two say it is necessary, even if a hundred say it is not necessary, and even if the sick person is with them that it is not necessary, we feed him because that two said it is necessary. Rema: And similarly, if a sick person and one doctor with him says it is necessary even if one hundred doctors say it is not necessary [Tur]. And we are not concerned that the sick person said it is necessary because he believes this (one) doctor that said it was necessary [Bais Yosef in the name of Mahria] (unlike paragraph 1 where the patient makes their own diagnosis)
ה אִם הַחוֹלֶה אוֹמֵר: אֵינוֹ צָרִיךְ, וְהָרוֹפֵא מְסֻפָּק, מַאֲכִילִין אוֹתוֹ; אֲבָל אִם הָרוֹפֵא אוֹמֵר: אֵינוֹ צָרִיךְ, וְהַחוֹלֶה אוֹמֵר: אֵינִי יוֹדֵעַ, אֵין מַאֲכִילִין אוֹתוֹ.
5 If the sick person says it is not necessary, and the doctor is in doubt, we feed him. But if the doctor says it is not necessary, and the sick person says 'I don't know', we do not feed him.
ו אִם הָרוֹפֵא אוֹמֵר שֶׁאֵינוֹ מַכִּיר אֶת הַחֹלִי, הֲרֵי הוּא כְּאָדָם דְּעַלְמָא וְאֵין דְּבָרָיו מוֹעִילִין וְלֹא מוֹרִידִין. הַגָּה: מִיהוּ אִם נֶחֱלַשׁ הַרְבֵּה עַד שֶׁנִּרְאֶה לְרֹב בְּנֵי אָדָם שֶׁאֶצְלוֹ שֶׁהוּא מְסֻכָּן אִם לֹא יֹאכַל, מַאֲכִילִין אוֹתוֹ (א"ו הָאָרֹךְ).
6 If the doctor says that he does not recognize the ailment, behold he is like any other person, and his words do not lift up or bring down (i.e. they do not affect the decision to eat or not to eat, which is based solely on the sick person themselves and expert doctors). Rema: However, if the person is very weak until it seems to most people that he is near danger if he does not eat, we feed him (despite not having a doctor to ask) [A'V HaAruch].
ז כְּשֶׁמַּאֲכִילִין אֶת הָעֻבָּרוֹת אוֹ אֶת הַחוֹלֶה, מַאֲכִילִין אוֹתָם מְעַט מְעַט כְּדֵי שֶׁלֹּא יִצְטָרֵף לְשִׁעוּר; הִלְכָּךְ מַאֲכִילִין אוֹתוֹ כְּב' שְׁלִישֵׁי בֵּיצָה בֵּינוֹנִית, וְיִשְׁהוּ כְּדֵי אֲכִילַת אַרְבָּעָה בֵּיצִים; וְהַשְּׁתִיָּה, יִבְדְּקוּ בַּחוֹלֶה עַצְמוֹ כַּמָּה הִיא כְּדֵי שֶׁיְּסַלְּקֶנּוּ לְצַד אֶחָד וְיֵרָאֶה כִּמְלֹא לֻגְמָיו.
7 When feeding a pregnant woman or sick person, we feed them a little at a time in order that it does not combine to the shiur (Halachic minimum amount to transgress the Biblical prohibition to eat). Therefore, we feed them like 2/3 of a medium-sized egg and wait (the amount of time) necessary to eat 4 eggs. And for drinking, the sick person checks themselves (i.e. prior to the fast) how much in order to fill one side (of the mouth), and it will be shown what a cheekful is (for that person which is the shiur for drinking).
ח וְיַשְׁקוּהוּ פָּחוֹת מֵאוֹתוֹ שִׁעוּר, וְיִשְׁהוּ בֵּין שְׁתִיָּה לִשְׁתִיָּה כְּדֵי אֲכִילַת אַרְבָּעָה בֵּיצִים, וּלְפָחוֹת יִשְׁהוּ בֵּין שְׁתִיָּה לִשְׁתִיָּה כְּדֵי שִׁעוּר שְׁתִיַּת רְבִיעִית; וְאִם אֲמָדוּהוּ שֶׁאֵין הַשִּׁעוּרִים הַלָּלוּ מַסְפִּיקִים לוֹ, אוֹ שֶׁהַחוֹלֶה אוֹמֵר כֵּן, אוֹ שֶׁנִּסְתַּפְּקוּ בַּדָּבָר, מַאֲכִילִים וּמַשְׁקִים אוֹתוֹ כָּל צָרְכּוֹ מִיָּד.
8 We give them to drink less from that shiur (see paragraph 7) and wait between each drink (the time) necessary to eat 4 eggs. Or at least, wait between each drink (the time) necessary to drink the shiur reviis (literally a 1/4th because it is 1/4 lug, another Halachic measurement, a reviis is appx 3-5 fl. oz.). Yet, if we calculate that these measurements are insufficient to him (the sick one) or the sick person says so (i.e. they are not enough) or even that we are uncertain in the matter, we give food and drink to him as necessary (i.e. whatever is sufficient even beyond the shiur of transgression) immediately.
ט מִי שֶׁאֲחָזוֹ בֻּלְמוֹס, וְהוּא חֹלִי שֶׁבָּא מֵחֲמַת רְעָבוֹן וְסִימָנוֹ שֶׁעֵינָיו כֵּהוֹת וְאֵינוֹ יָכוֹל לִרְאוֹת, מַאֲכִילִין אוֹתוֹ עַד שֶׁיֵּאוֹרוּ עֵינָיו; וְאִם אֵין שָׁם מַאֲכָל שֶׁל הֶתֵּר, מַאֲכִילִין אוֹתוֹ מַאֲכַל אִסוּר; וְאִם יֵשׁ כָּאן שְׁנֵי מִינֵי אִסוּרִים, אֶחָד חָמוּר מֵחֲבֵרוֹ, מַאֲכִילִין אוֹתוֹ הַקַּל תְּחִלָּה. הַגָּה: אִם צָרִיךְ לְבָשָׂר וְיֵשׁ כָּאן בְּהֵמָה שֶׁצְּרִיכִין לְשָׁחֲטָהּ וּבְשַׂר נְבֵלָה מוּכֶנֶת, עַיֵּן לְעֵיל סִימָן שכ"ח י"ד.
9 One who is grasped with (the illness of) bulimus, and this sickness comes from starvation, its signs are his eyes dim and is unable to see, we feed him until his eyes lighten. And if there isn't there permitted food (i.e. kosher), we feed him forbidden food (non-kosher). And if there are two types of forbidden (food) - one stricter than its friend (i.e. one is Torah mandated, one Rabbinic, or one has a more strict Torah punishment for wanton eating of it), we feed the more benign first. Rema: If it is necessary meat (for a sick person), and here there is a (kosher) animal that needs to be slaughtered or prepared neveilah meat (i.e. from an animal that died NOT from proper ritual slaughter), see prior Siman 328 Paragraph 14.
י חוֹלֶה שֶׁאָכַל בְּיוֹם הַכִּפּוּרִים וְנִתְיַשֵּׁב דַּעְתּוֹ בָּעִנְיָן שֶׁיָּכוֹל לְבָרֵךְ, צָרִיךְ לְהַזְכִּיר שֶׁל יוֹם הַכִּפּוּרִים בְּבִרְכַּת הַמָּזוֹן, שֶׁאוֹמֵר: יַעֲלֶה וְיָבֹא, בְּבוֹנֵה יְרוּשָׁלַיִם.
10 A sick person that eats on Yom Kippur, and calms down in a matter that they are able to make blessings, it is necessary to mention it is Yom Kippur in Birkas Hamazon (Grace after Meals) that he says in the "Yaaleh v' Yavo" (May there arise and come etc., a section that is mentioned on Biblical Festivals) in (the blessing of) "Boneh Yerushalyim" (who builds Jerusalem).