It is common knowledge that anything ingested during fasting period is not acceptable. But what about things getting inside the body by other means? Can injections be taken while fasting?
Summary: Surveying online fatawa, I found two main different opinions:
Nourishing injections break the fast, whereas non-nourishing medical injections do not.
Injections do not break the fast because they do not enter the body "through an open passage".
Injections for nourishment
One type of injection which gives a person a nourishment, which is equivalent to giving food. For example, it you take an intravenous glucose for people who have excessive dehydration, etc., and cannot take food by the mouth. Normally the medical people is giving intravenous glucose. This is a form or nourishment. It is equivalent to giving food; this will break the fast.
Injections not for nourishment
But if it's not for nourishment, if it's only a medical treatment, like insulin, it can be subcutaneously, or penicillin, all these medical treatments which are not nourishment to the body are not [??] like food, these can be given during the daytime and the fast will not break. There are some scholars who say it's makruh (or discouraged). It's preferable if it's taken after the sunset but the right ruling is that it does not break the fast, because it's not in [??] of food.
Dar al-Ifta'a differed on this, writing:
The ruling on using injections, saline solutions, glucose and mannitol is that they all do not spoil fasting because they do not enter the body through an open passage but through the skin which absorbs them. Hence, there is no difference between getting them via skin absorption or injection.
In one in need of medical attention takes an injection, it would not invalidate his fast in any way.
Ayatollah Sistani writes:
There is no objection to an injection which anaesthetises one's limb or is used for some other purpose being given to a person, who is observing fast, but it is better that the injections which are given as medicine or food are avoided.
Ayatollah Khameni (leader.ir) writes:
It is based on obligatory caution for the fasting person to avoid having any kind of supportive, nutritional or intravenous injections. The same rule is applied to all kinds of intravenous fluid infusions. However, there is no objection to using anesthetic injections and intramuscular ones for treatment purposes.
and also "Injections that are used in place of food invalidate the fast." (source: Al-Islam)
The medicine injection in a muscle or a vein does not breaks one's fasting because the medical substance does not enter the belly directly by a normal open access. However, if the injection is nourishing, i.e., it substitutes eating and drinking, then it breaks the fast.