What is the theme in Under the Volcano by Malcolm Lowry?

The story in this novel centers on the consul's alcohol addiction. It appears to be a problem he struggles with for years on end. In fact, the author insinuates that the addiction causes Yvonne to leave him one year prior to the novel's beginning. As often happens with addiction, this loss fails to motivate the consul to stop drinking. Instead, as Laruelle points out, the consul's drinking worsens.