In the case of intravenous and intramuscular, nature of medication plays a major role. Actually, both can serves a rapid relief but depends on the medication to be administered as some medications contradict the other.

There are medications specifically made for intravenous (IV) while some are made specifically for intramuscular (IM), also some medications are made using the both( intravenous or intramuscular).

These are due to their chemical components and the adverse effects on patients.