Praise be to Allah.
An injection that does not provide nourishment does not break the fast, as we have explained in the answer to question no. 49706.
If the one who is having this injection needs a lot of water and food afterwards, then we should examine the matter further. If it is possible to delay the injection until after iftar without causing any harm or hardship to the patient, then this is what must be done.
If delaying it until after iftar will cause him harm, such as making his illness worse, then he must have the injection and break the fast on the day that he has it.
If delaying it will not harm him, but it will cause him hardship, then it is mustahabb for him to break the fast, and it is makrooh for him to fast.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: There may be several scenarios in the case of one who is sick:
The first is where he is not affected by fasting, as in the case of a slight cold, slight headache, toothache and the like. In this case it is not permissible for him to break the fast. Even though some scholars say that it is permissible for him to do so, because of the verse (interpretation of the meaning), “and whoever is ill” [al-Baqarah 2:185], we say that this ruling is connected to a reason, which is when breaking the fast is kinder to him. In that case, we say to him that breaking the fast is better. But if he will not be affected, then it is not permissible for him to break the fast, and he must fast.
The second scenario is where it is difficult for him to fast, but it will not harm him. In this case it is makrooh for him to fast, and it is more appropriate for him not to fast.
The third scenario is where it is difficult for him to fast and doing so will harm him, such as a man who is suffering from kidney disease or diabetes and the like, and fasting will harm him. In his case, fasting is haraam.
Thus we will realize the error of some people who are willing to try hard, and some sick people for whom fasting is difficult and may harm them, but they refuse to break their fast.
We say: these people are mistaken, because they have not accepted the kindness of Allah, may He be glorified and exalted, and they have not accepted the concession that He granted, and they are harming themselves. Allah, may He be glorified and exalted, says (interpretation of the meaning): “And do not kill yourselves” [an-Nisaa’ 4:29].
End quote from ash-Sharh al-Mumti‘ (6/352).
And Allah knows best.