What is the theme in The Ruby in the Smoke by Philip Pullman?

Opium is a very important theme in 'A Ruby in the Smoke' as it pulls all the different plot elements together. Almost all the suffering and evil portrayed in the story is somehow caused by opium. It is represented as a very dangerous, destructive and evil substance, which will only cause greed, misery and corruption. Many of the characters declare their disgust for and hatred of the drug, including Lockhart, Reverend Bedwell and Frederick.