Someone who is sick is permitted to swallow medicine (without water or washed down by an inedible, bitter liquid) on fasts other than Yom Kippur if the medicine is not normally edible.[1] According to several opinions, this is permissible even on Yom Kippur.[2] There are two complicating factors regarding a caffeine pill:
Caffeine is not a medicine that heals. Rather it is an ingredient in regular foods that is missing during a fast.
One usually takes this pill when perfectly healthy to prevent a future decline in his condition.
(The following guidelines do not apply to Yom Kippur.) However, the above considerations don’t preclude the use of caffeine pills, just limit it. Caffeine withdrawal is not a lack of nutrition but a disease-like situation caused by an acquired dependency upon caffeine. When, based on experience, one expects to feel ill to the extent that it will affect his ability to function, as is the case with a severe headache, he can take the pill in advance to prevent deterioration.[3] If he is affected enough to be confined to bed, then he could take the pill with water if needed.[4] If only a mild headache is expected, then he should not take the pill. After all, healthy people don’t always feel great on a fast day. If one is likely to get a bad headache but can prevent it by taking the pill when the symptoms begin, he should delay taking it until it becomes evident that a serious headache is coming on. In general, in borderline cases, there is more reason to be stringent on Tisha B’Av than on the other fasts.[5]
Some people solve the problem by avoiding caffeine all year long or by slowly weaning themselves during the weeks before the fast. If these solutions are practical, they are also preferable.