what's the difference? synonymous is an adjective, and synonym is a noun, the meaning's the same... however, these two words are not synonyms in any way for sure...
1. a woman possessing heroic qualities
2. a woman idealized for possessing superior qualities
3. the main female character in a novel, play, film, etc.
Anyone with a mind not clouded with bias and spite can see that this usage of heroine is what actually fits with the rest of the lyrics.
especially coming together with lines like 'I can feel you in my veins', haha... it's probably more likely that the woman that comes to your mind while getting through some common collocations, is exactly kind of a hidden image of the substance and nothing else,.. It's a cheap way to sing about the addiction, like it would be singing about love, but with pictures coming from the metaphors of love and addiction... Anyway, I'm not sure cause I didn't focus on it too much - I'm glad I'm not an english native speaker and if I want to I can listen to anything without hearing the context ;p