Adios

It is obviously about Herion, this seems to be correct because herion is known to give you constipation really bad and cause halluciantions.

He puts the needle inside the vein (There goes the herion)

Asks the music to come in (Herion usually lightens the mind, in Tills case maybe music would let him rest)

Between his throat and forearm (The inside bend of the arm (usually were you shoot up) or possibly his heart)

The melody goes softly through the bones (He has the sweet feeling on the drug taking place)

He has his eyes closed (Then the hallucinations come)

In his blood a battle rages (Probably freaks the hell out of him with pain)

An army marches through his bowel (the constipation)

The intestines slowly become warm (When the herion has run its course your bowels loosen thus you need to take a crap really bad)