Rav Moshe Feinstein was asked about a woman who reacted to Yom Kippur by becoming faint. Her doctors suggested that she take some form of vitamin suppository that would prevent her from having such a condition and would thus allow her to continue her fast. Are such suppositories allowed on Yom Kippur?
Rav Moshe begins his response by analyzing the seriousness of the condition. According to the Gemara, someone who is seized by bulmus can eat - the question is what exactly that condition is and whether this condition is comparable. Rav Moshe reasons that bulmus is a state of illness that afflicts an otherwise healthy individual as a result of the fast, and that bulmus can be perceived by a marked change in a person's eyes. However, if such a change does not occur then we would simply say that the individual is having a difficult fast, which is a normal occurrence and does not lead to permission to eat. This case of the fainting woman may be similar in that we might allow her to eat given her condition.
That being said, can the woman opt to continue to fast by take vitamins rectally? Rav Moshe considers the possibility that since the vitamins are not actually curative they might run afoul of the Rabbinic prohibition of שחיקת סממנין, grinding medicinal plants, which is the source for our general prohibition of taking medicine on Shabbat. However, Rav Moshe reasons that since an individual would be allowed to take food through a passageway other than the mouth, then certainly this vitamin capsule should be allowed without worrying about violating the decree of שחיקת סממנין.