It's about a friend of his who OD'd (and died I think?) on his 21st birthday. I only know that because I read some intro he gave to it, saying "I wrote this for a friend of mine, named so-and-so, who on his 21st birthday gave himself the present of enough heroin in his bloodstream to kill him." don't really know the whole story though...
anyway, even though he had a specific person in mind, it's definitely a universal song about addiction, not even to a drug necessarily, but even in a relationship, letting someone go...