The Shulhan Aruch, in discussing the laws of fast days (Orah Haim 566), rules that somebody who must eat on a fast day may not receive an Aliya to the Torah. The question arises as to whether this applies also on Yom Kippur, if a patient or elderly person is advised by doctors that he must eat or drink, and, after consulting with his Rabbi, he eats or drinks "Pahot Pahot Mi’ke’shi’ur" – meaning, small amounts at a time, as required by Halacha. Such a person clearly acts in full accordance with the Halacha, as he is allowed (and in fact required) to eat to maintain his health. If, for example, such a person purchased an Aliya in the synagogue, may he be called to the Torah? This question also arises in the case of somebody who requires medication, and takes a sip of water to swallow his pill on Yom Kippur. May he receive an Aliya to the Torah on Yom Kippur?
Rav Haim Palachi (Izmir, Turkey, 1788-1869), in several places in his writings (Leb Haim, vol. 3 Siman 134; Mo’ed Le’chol Hai, Siman Tob, #8), rules that one who eats or drinks on Yom Kippur may not be called for an Aliya, even though he eats or drinks in accordance with the Halacha, just like on other fasts. Rav Haim Palachi notes that the Yom Kippur Torah reading speaks of the requirement of Inui (afflicting oneself), and it is therefore inappropriate for a person who is not observing this command – even if he is legitimately exempt – to receive an Aliya to the Torah.
However, the Hatam Sofer (Rabbi Moshe Sofer of Pressburg, 1762-1839), in his responsa (157), as well as Rabbi Akiva Eger (1761-1837), in his Hiddushim (24), rule leniently on this issue. They note a fundamental distinction between the Torah reading on ordinary fast days and the reading on Yom Kippur. On ordinary fast days, the Torah is read solely because of the fast; it is the observance of a fast day that obligates the congregation to read the Torah. Therefore, it stands to reason that a person who, even for legitimate reasons, is not participating in the fast may not participate in the congregational Torah reading by receiving an Aliya to the Torah. On Yom Kippur, however, the Torah reading is required by force of the day’s status as a Yom Tob. Just as we read the Torah on Shabbat, Pesah and other Yamim Tobim, we read the Torah on Yom Kippur, as well. On Yom Kippur, then, the Torah reading is not dependent upon the fast specifically; it is required due to the occasion of the Yom Tob. Therefore, on Yom Kippur, somebody who does not fast may nevertheless receive an Aliya to the Torah.
Hacham Ovadia, in Hazon Ovadia (p. 349; listen to audio recording for precise citation), follows this lenient ruling of Rabbi Akiva Eger and the Hatam Sofer, and allows someone who is not fasting on Yom Kippur to receive an Aliya. In addition to the aforementioned line of reasoning, Hacham Ovadia notes that if a person eats small quantities at a time, as is required, then he is still considered observing the requirement of Inui, and thus he may certainly be called to the Torah for an Aliya.
Hacham Ovadia adds that an elderly Hazzan who is frail and must drink small amounts of water on Yom Kippur is allowed to serve as Hazzan, even though he drinks. This would certainly apply to a Hazzan who must swallow a pill and therefore takes a small sip of water. Even though he ingests a small amount of water, he may nevertheless serve as Hazzan on Yom Kippur.
Summary: One who eats or drinks on Yom Kippur on the basis of a doctor’s instructions, after consultation with a Rabbi, may nevertheless receive an Aliya to the Torah on Yom Kippur and serve as Hazzan. This applies as well to somebody who must take a pill and therefore sips a small bit of water to swallow his pill.