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Q: What is the halacha for yom kippur? I am pregnant, and the doctor said I should definitely eat/drink if I start to feel faint. If I do need to have something, what should I have and how do I measure the amount?
A: Drinking: Fill up your mouth with as much water as it can possibly hold, then expel all of the water into a disposable cup. Empty half of the water into a second cup. Slightly less than this quantity is the amount you can drink each time, so make a mark slightly below the level of the water on one of the cups with a marker (R. Yosef Zvi Rimon). You can drink this amount (of either water or fruit juice, etc.) every 4 or 5 minutes, and if more is needed after only waiting 2 or 3 minutes.
If you need to eat, you can eat the amount of food that equals the approximate volume of a small box of matches (about 1 fl. oz./30 ml) every 9 minutes. If necessary, it may be eaten every 4.5 minutes.
Eating and drinking don’t combine, so you can do both in the same time period.
If one has a medical need to consume more than the amounts above she should eat and drink normally.
Q: I need to swallow medication every day. What do I do on Yom Kippur?
A: If possible, it should be swallowed without water. One may also use a small amount of a liquid that is not normally considered food to swallow the pill, for example water mixed with a bitter substance (like highly concentrated tea essence), mouthwash, lemon juice diluted with a small amount of water, salt water, or water with a drop of liquid soap added to it. If one must take the medicine in order to prevent the development of a serious medical condition, and certainly if one suffers from a potentially life threatening situation, it may be taken with a bit of water.
Source: Rabbi Isaacs Blog