Opioids are robust analgesic medications that have played a core role in the management of acute and chronic pain for decades. The term opioid is used for drugs that are derived from the opium plant Papaver somniferum. The term narcotic is derived from the Greek word “narcosis” (meaning stupor), which, in legal context, includes opium derivatives, their semisynthetic substitutes, and certain stimulants such as cocaine. Thus, all opioid medications are narcotics, but all narcotics are not opioids. Anesthesiologists constantly deal with opioid management for their patients in the perioperative setting. This frequently involves changing one opioid to another or changing the delivery route of a given opioid. The complex pharmacology of these drugs, coupled with significant interpersonal variation seen clinically with these medications, often makes this job cumbersome. Mastering the art of these challenging conversions requires understanding some of the basic pharmacology of these drugs.

Opioid medications are delivered by all possible routes: topical, transdermal, transmucosal, oral, rectal, and parenteral (subcutaneous, intramuscular, or intravenous [IV]). They bind to Mu, Delta, and Kappa types of opioid receptors in central and peripheral nervous system to cause analgesia or certain untoward side effects. Based on their affinity to Mu-receptors, there is a considerable divergence among these medications. Fentanyl, for instance, is almost 15 times as potent as hydrocodone, which in turn is 4 to 5 times stronger than morphine for its analgesic properties. Moreover, there is a significant degree of interpersonal variability for any given opioid, depending on the type of pain, psychological factors, therapeutic drug interactions with other medications, age (elderly are considered to be more sensitive), certain pathophysiological states (increased sensitivity with central nervous system comorbidity), hepatic and renal dysfunction, and even genetic variations. Thus, 2 to 4 mg of IV morphine may not provide adequate pain control in a young patient after shoulder arthroscopy but may produce sedation and respiratory depression in an elderly man with arthritis pain. A brief outline of pharmacokinetic properties of commonly used opioids is given in Table 164.1.

To resolve some of these complex issues, Houde et al. and Bruera et al. performed single-dose relative potency studies on opioid medications. Based
on these relevant studies, an equianalgesic dose table (Table 164.2) has subsequently been formulated that describes relative potencies between these diverse opioids for both oral and parenteral routes of administration. The term equianalgesic is used for two doses with comparable pharmacologic analgesic effects, either of different drugs or of the same drug but with different routes of delivery. Thus, the first two columns of Table 164.2 list the oral and parenteral doses of opioids that are equivalent to 10 mg of parenteral morphine. These values essentially account for pharmacodynamic and pharmacokinetic differences among these drugs. Table 164.2 is a stepping stone in solving complicated issues related to opioid management.