Opioids refers broadly to all compounds related to opium that possess analgesic and sedative properties. Opiate describes the alkaloids codeine and morphine found naturally in the opium poppy plant, Papaver somniferum. Narcotic refers to a broader group of soporific drugs, a term predominantly used by law enforcement to refer to controlled substances with abuse or addictive potential; use of this term in medical practice is discouraged.
Opioid abuse is a significant public health issue in the United States with a dramatic increase in ED visits related to opioids in the past 20 years.1,2 Opioids were responsible for 66% of drug overdose deaths in the United States during 2016.3 The majority of prescription opioid overdose deaths were associated with diversion, doctor shopping, and nonmedical use.
Opioids modulate nociception at the terminals of afferent nerves in the CNS, peripheral nervous system, and GI tract. Opioids are agonists at the three primary opioid receptors: µ (mu), κ (kappa), and δ (delta). Opioid receptors are similar to other G protein-coupled receptors; transmembrane proteins that undergo conformation change when activated by external molecules that then alters some aspect of intracellular function. Opioid receptors vary widely in morphology and distribution. In addition, the specificity and affinity of an opioid for a particular receptor are variable. For example, tramadol possesses 1/6000th the affinity of morphine at the µ-receptor site.
Stimulation of the µ-receptors results in analgesia, sedation, miosis, respiratory depression, cough suppression, euphoria, and decreased GI motility. Stimulation of κ-receptors results in weaker analgesia, sedation, miosis, decreased intestinal motility, dysphoria, and hallucinations. Stimulation of the δ-receptors results in some analgesia and antidepressant effect. All currently available opioid agonists possess µ-receptor activity and result in some degree of respiratory depression.
Opioids can be categorized as naturally occurring compounds (termed opiates), chemical modifications of natural compounds (semisynthetic), and the synthetic derivatives (Table 186-1). Some opioids are agonists at all opioid receptors (e.g., morphine and hydromorphone), whereas others are partial agonists–antagonists (e.g., pentazocine, butorphanol, nalbuphine, and buprenorphine) at the opioid receptors.
TABLE 186-1Classification and Characteristics of Major Pharmaceutical Opioids ||Download (.pdf) TABLE 186-1 Classification and Characteristics of Major Pharmaceutical Opioids
| ||Oral Dose Equianalgesic to Morphine 10 milligrams SC (milligrams) ||Parenteral Dose Equianalgesic to Morphine 10 milligrams SC (milligrams) ||Duration of Analgesic Action* (h) ||Elimination Half-Life* (h) |
|Codeine ||200 ||120 ||4–6 ||2.5–4 |
|Morphine ||30 ||10 ||3–4 ||2–4 |
|Buprenorphine ||4 SL ||0.3 ||6–24 ||20–44 |
|Hydrocodone ||30 ||Not available ||4–6 ||8 |
|Hydromorphone ||7.5 ||1.5 ||2–4 ||2–3 |
|Oxycodone ||20 ||Not available ||3–6 ||3–4 |
|Oxymorphone ||6 ||1.5 ||4–6 ||7–11 |
|Diphenoxylate ||2.5 ||Not available ||Not applicable ||2 h for diphenoxylate and 12–14 h for difenoxin† |
|Fentanyl ||0.125 ||0.100 ||1 ||3–4 |
|Meperidine ||300 ||100 ||1–3 ||3–4 h for meperidine and 15–30 h for normeperidine† |