The song is basically about Heroins addiction, but in my opinion it also encompasses a whole lot more.
Undoubtedly one of Bono's finest moments, the song encompasses feeling relating to Love, Hate and War.
He is reaching out to an addict. The addict needs help and knows he/she needs the help, but they are in the situation where their only desire in life is where their next fix is coming from, it dominates their every action, and the lethargy of it is a cancer, what they need most and live for is killing them.
The state they are in prevents the singer from helping them, and he is expressing his pain and helplessness , and his inability to get throuhg or help.
Personally I think the verse
If you should ask then maybe they'd
Tell you what I would say
True colors fly in blue and black
Bruised silken sky and burning flack
Colors crash, collide in blood shot eyes
contrasts the pain with the troubles in Belfast in the 80's.