No, it definetly is not about girlfriends, I believe you have percieved this remarkable lyric incorrectly, unless Matt Bellamy is actually addicted to his own girlfriend.
Obviously his girlfriend counts as someone who does care about him a lot, why do you think he said "people who care if I live or die"?.
Rather this is a song dedicated to drug addicts, and its actually trying to make people realise the power drugs can have you, and the people around you.
Nothing could ever change your world for the worse than drugs can (except health concerns and other shit).
He also makes a reference to depressed people who feel so awful that they take drugs in a desperate attemtp to cope with pain or maybe addicted people who are so desperate for drugs, he says in his lyrics "All the souls that would die just to feel alive". Drugs have the power to change your mood, and therefore has the power to hook you.
But he isn't the only generous artist to do this Michael Jackson created lyrics for drug addicts to, try looking at "Beat It"