Laudanum, a mixture of alcohol and opium used as a common medicine in the nineteenth century, is central to the mystery of the Moonstone. And it takes an opium addict Ezra Jennings to figure it out. What many readers don't realize is that Wilkie Collins was an opium addict himself – he took laudanum regularly to treat the pain of his rheumatic gout (a disease that causes a tremendous amount of pain in the joints). Besides all this drug use, there's a lot of other addiction in The Moonstone, as well – Franklin Blake is addicted to tobacco, and he gets insomnia when he tries to quit cold turkey.
Opium was commonly believed to have been imported to Britain from China and India; it seems appropriate, therefore, that the theft of the Indian diamond should be caused by the unconscious consumption of this foreign substance.
Addiction is not associated with weakness of character in The Moonstone; rather, the two addicts of the novel, Franklin Blake (tobacco) and Ezra Jennings (laudanum) are two of the most admirable characters.