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Even with proper pre-fast hydration and caffeine reduction, I almost always get a headache starting around hour 20-22 of a fast and growing from there. Taking a nap is sometimes possible and sometimes helps a little, but not always. (And on Yom Kippur I sure don't want to risk oversleeping and missing part of neilah.)

Is there anything else I can do on the fast day when the headache starts to reduce its effects?

(Even if I had a heter to take medicine (which I don't), all painkillers that I'm aware of do bad things when taken on an empty stomach, and at that point in the day the stomach is quite empty.)

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I think I've heard of people taking a slow-release caffeine pill right before the fast, but I don't know what it's called, or whether it's legal and safe. – Isaac Moses Sep 25 '12 at 16:19
I've heard of such products but never seen this for sale (in the US). I'm also mindful of the article linked in this answer to a question about thirst warning against drinking coffee right before the fast. (Caffeine is a diuretic, after all.) – Monica Cellio Sep 25 '12 at 16:23

CYLOR, but I have heard of people taking caffeine suppositories on Yom Kippur.

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Oh my. I had no idea such things existed! – Monica Cellio Sep 27 '12 at 3:35
Oy​​​​​​​​​​​​​​​. – Double AA Sep 27 '12 at 5:53
also CYLGI (See your local gastrointerologist)... – Charles Koppelman Sep 27 '12 at 18:28

How are you preparing for the fast?

The 4 worst things to eat before a fast:

  1. Cheese and dairy
  2. Eggs
  3. Meat
  4. Potatoes and sugar

And I bet a lot of people eat those things. The problem with the first three is the high protein (and high salt in the case of cheese) require a LOT of water to digest, this makes you thirsty and dehydration causes headaches.

Potatoes and sugar digest extremely fast, which leave you hungry quickly. (To the body potatoes are basically indistinguishable from sugar despite being called vegetables.)

The best things to eat are:

  1. Complex carbs
  2. Fiber
  3. A small amount of fat/oil

Beans/Lentils and other legumes and whole grain products are ideal. Bread is good too. I would add a small amount of fat (whatever you would normally use to cook with). (So a bean/barley/pea soup is perfect - but don't overdo the salt of course.) Salad is surprisingly good - lots of water and fiber (but don't overdo it, you need some calories too). Cooked vegetables should round out the meal.

Do not try to stuff yourself with calories - you need less than you think. What you need to worry about it water.

Next is pre-fast preparation:

How much water do you normally drink during the day? On a fast day your body will expect that much. If you normally drink 8 cups a day you will be in trouble. You need to start reducing your water intake a week before the fast starts.

Basically you want your urine to always be yellow (light yellow is OK) before the fast. Then your body is accustomed to conserving water and will do well with the extra you give it before the fast.

But you can't just drink water - it'll come right out again. You need to mix it with food to slow down the absorption. Your blood is unable to hold much water, you must hold it in your stomach and intestines, and the only way to do it is to absorb it into food - that's why salad is good. So is soluble fiber.

It should go without saying that you should avoid all caffeine for a week before the fast.

For further reading see this study: http://www.ima.org.il/FilesUpload/IMAJ/0/60/30277.pdf (Which, reading between the lines, appears to have been done on frum people.)

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Thanks for the advice. Your answer might be a better fit here, which is a question about avoiding thirst. My question is about dealing with the headache that, at least for me, seems to show up no matter what I do in advance. (I've tried a bunch of different approaches, including what you describe, but I always seem to get the headache anyway. So while I figure out prevention I'd like to know how to deal with it when it shows up.) – Monica Cellio Sep 27 '12 at 3:34
@MonicaCellio Headaches (usually) come from two things: Thirst and caffeine withdrawal. If you skip breakfast and maybe lunch too on an ordinary day, but drink water and have coffee do you get a headache? (If you don't know, then sounds like an experiment is in order.) – Ariel Sep 27 '12 at 3:49
I drink a lot of both water and caffeine over the course of a typical day, and don't always get breakfast and lunch. I don't get headaches on those days. I don't know if that's due to staying hydrated or the caffeine maintenance. I ramp the caffeine down to zero in advance of Yom Kippur, BTW. (Why don't I keep it there and give up caffeine? Sleepy! :-) ) So I've been assuming that the YK headache is primarily due to dehydration, though maybe I'm kidding myself that a week of caffeine reduction helps. – Monica Cellio Sep 28 '12 at 2:42
@MonicaCellio In that case your headache is not from not eating, it's from dehydration and caffeine. Are you at zero caffeine for at least 3 days before Yom Kippur? And follow my pre-fast plan to help with the dehydration. – Ariel Sep 28 '12 at 3:09
I will say that there was a fast day where I saw midway through that my then small headache would inevitably become a big headache, so I had one small cup of coffee. I felt great for the balance of the day. Apparently, my headache comes from caffeine (and maybe dehydration). – Ze'ev Felsen May 24 '13 at 4:14

Rav Chizkiyahu Nevenzahl Shlit"a, current Chief Rabbi of the Old city permits swallowing pills without water on a fast day (including Tisha B'av and Yom Kippur) because swallowing is not considered derech achila - the normal way of eating, of which is prohibited on a fast day .

This is not to be relied upon for practical halacha

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