The Official Website of Grand Ayatollah Makarem Shirazi

Not all sicknesses exempt one from fasting. For example, some sicknesses can be controlled by taking the necessary medication before a fajr and after iftar. In such cases, it is obligatory for one to fast. But if this will harm their bodies, they must not fast. It is also possible that one feels weak due to intense activity or hard work while one is fasting. But this does not allow one to break their fast unless their loss of energy or their thirst becomes unbearable, harmful or even nearly fatal. In such cases, one can eat or drink water a little but they must not eat or drink any more during the time of fasting and if this has happened during an obligatory fasting, they must also observe a qada fast later if they can. Sometimes, a fasting person who is sick feels better or is cured by an intramuscular injection but it is impermissible to receive intravascular shots and if they receive blood transfusion, they must complete their fasting on that day and then, according to the obligatory precaution, they must also fast another day as compensation for that day's fasting. However, as mentioned earlier, it is permissible for them to receive any sort of intramuscular shots and it will not invalidate the fast. Also, using suppositories will not invalidate fasting but according to precaution, it is considered abominable during fasting. It might also be necessary for some to undergo dialysis during fasting. In such cases, if something is taken away from their bloodstream, their fasting will be valid. But if they undergo exchange transfusion or something is added to their bloodstream, the obligatory precaution demands that they observe the qada fast of that day as well. Visiting the dentist is also permissible during fasting but one must be careful not to swallow anything. If one gets a filling and they are certain that getting a filling will not cause them to swallow blood and they inadvertently swallow some blood, their fasting will not be invalidated. Having ultrasonography diagnostic tests does not invalidate fasting. Also, having endoscopy or colonoscopy diagnostic tests does not invalidate fasting as long as no lubricants or other such substances has been applied on their devices. Using eye d r o ps will not invalidate fasting either as long as the d r o ps do not find their way into the mouth or are not intentionally swallowed. But using ear d r o ps or nose d r o ps, which one is sure will find their way to the throat and will be swallowed, is problematic while fasting.