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Opioids are naturally occurring, semisynthetic, and synthetic drugs whose effects are mediated through opioid receptors, altering the perception and emotional response to painful stimuli [1, 2]. There are three major classes of opioid receptors: µ (mu), κ (kappa), and δ (delta), with subtypes within each receptor class [3]. Opioid receptors are found in the periaqueductal grey matter and throughout the spinal cord, as well as nonneural sites such as joint synovium and intestinal mucosa [1,2].

With approximately 80% of visits to physicians involving conditions with a painful component and at least 50 million Americans suffering from chronic pain, it is imperative to have an appreciation of pharmacotherapies available for treating pain [1,2]. Pain medications account for the second most prescribed class of drugs (after cardiac-renal drugs), making up 12% of all medications prescribed during ambulatory office visits in the USA [3]. The National Institutes of Health have estimated chronic pain to be the third largest health problem in the world [4]. Chronic pain is a problem that will become increasingly more prevalent as the US population aged 65 and older rapidly increases, as it is predicted to exceed 70 million by the year 2030 [2]. As many as 80% of older persons with cancer experience pain during the course of their illness [5]. The first chapter of this book makes it clear that complaints of pain are particularly common among older adults, occurring twice as often in persons over 65 years of age as compared to younger people [6–8].