noun which may take the genitive case particle 'no', noun
intramuscular
Medical term used to describe something located or occurring within a muscle, such as an injection.
この薬は筋肉内注射で投与されます。
This medication is administered by intramuscular injection.
静脈内 means intravenous (within a vein), a different route of administration.
Compound of 筋肉 (muscle) + 内 (inside). The reading is the standard on'yomi of the kanji.