May one take medicine on a fast day?
If one is sick, then he is not required to fast, as explained above. Even if one is not sick, it is permitted to take medicine if the medicine is bitter. He is to swallow it without water or alternatively place something bitter in the water and use this bitter water to help swallow the pill. [If one is unable to do so, then the medicine may be taken with regular water if the person will fall sick if he does not take it.]
 Beir Heiytiv 567:7 in name of Dvar Shmuel; Halachos Ketanos 97; Kaf Hachaim 554:34 in name of Kesonos Yosef 4; Ikarei Hadaat 29:36; Tosefes Chaim on Chayeh Adam 1:135-8; Pischei Olam 554:6; Piskeiy Teshuvos 554:7; 567:3; See also regaridng Yom Kippur: Sdei Chemed Yom Kippur 3:8; Yeshuos Yaakov 612; Kesav Sofer 111; Pischeiy Teshuvah Y.D. 155:6; Shoel Umeishiv Mahdura Daled 1:55; Orchos Chaim 618:1; Eretz Tzevi 88; Igros Moshe 3:91; Tzitz Eliezer 10:25; SSH”K 39:8; Nishmas Avraham 612:7; Piskeiy Teshuvos 612:2; Nitei Gavriel 37:23; 39:12-15
 Igros Moshe 3:91; Piskeiy Teshuvos ibid