Two scenarios may apply to having injections during the day in Ramadaan:
1 – Where the injection provides nourishment that takes the place of food and drink; this kind of injection invalidates the fast because it is like eating and drinking.
2 – Where what is injected is not a kind of nourishment; this does not break the fast or affect it. In this case it makes no difference whether the injection is into a vein or a muscle.
But if it is possible to have these injections at night, that is better and more on the safe side with regard to the fast.
Shaykh Ibn Baaz (15/257) was asked about the ruling on a person who had intravenous and intramuscular injections during the day in Ramadaan when he was fasting and completed his fast – was his fast invalidated and does he have to make it up, or not?
His fast is valid, because an injection into a vein is not like eating or drinking; this applies more so in the case on an intramuscular injection. But if he makes it up in order to be on the safe side, that is better. If it can be delayed until nighttime, if he needs to have it, that is better too, so as to avoid an area of scholarly dispute.
Shaykh Ibn ‘Uthaymeen was asked in Fataawa al-Siyaam (p. 220) about the ruling on having injections into a vein, muscle or the buttock.
There is nothing wrong with having injections into a vein, muscle or buttock, and that does not break the fast, because this is not one of the things that break the fast; it is not food or drink, and it is not like eating or drinking. We have already explained that this does not have any effect on the fast. What does affect the fast is when a sick person is given an injection of something that takes the place of food and drink.
The Standing Committee (10/252) was asked about the ruling on administering medicine via injection during the day in Ramadaan, whether that is nourishment or medication.
They replied: it is permissible to administer medicine by needle into a muscle or vein to a fasting person during the day in Ramadaan, but it is not permissible for the fasting person to be given a nourishing injection during the day in Ramadaan, because that comes under the ruling on eating or drinking, so that injection is regarded as a means of breaking the fast in Ramadaan. If the injection can be given into a muscle or vein at night, that is better.