I caught you knockin’ at my cellar door: I love you, baby, can I have some more? Ooh, ooh, the damage done.
I hit the city and I lost my band: I watched the needle take another man. Gone, gone, the damage done.
I sing the song because I love the man! I know that some of you don’t understand [how you need to] milk-blood* to keep from running out.
* “milk-blood” most likely refers to the act of extracting heroin-laden blood, for reinjection at a later time. It usually is one’s own blood, but could also be the blood of someone who has just overdosed. It is done as “insurance” in case one’s heroin supply runs out. [source]