Praise be to Allah.
It is permissible for a fasting person to take medicine by injection, whether intramuscular or intravenous, during the day in Ramadaan, but it is not permissible for the fasting person to be given an injection for nourishment purposes during the day in Ramadaan, because that comes under the ruling on eating and drinking; having this injection is regarded as a trick aimed at breaking one's fast in Ramadaan. If the intramuscular or intravenous injection can be given at night, that is better.
And Allaah is the Source of strength. May Allaah send blessings and peace upon our Prophet Muhammad and his family and companions.