The World Federation One Stop Fiqh

Ruling 1560

If a fasting person becomes so thirsty that he fears he may die of thirst, sustain some harm, or fall into hardship that he cannot bear, he can drink water to the extent that his fear of these things is averted; but in this case, his fast becomes invalid. In fact, in the case of fear of death and suchlike, it is obligatory for one to drink. If it is the month of Ramadan, then based on obligatory precaution, the person must not drink an amount that is more than necessary, and for the rest of the day he must refrain from doing anything else that invalidates a fast.