Opioid Pharmacology

There exist now a wide variety of choices for narcotic therapy. Differences in side effect profiles have been attempted to be quantified, but it appears that the majority of differences in efficacy and side effects are very patient dependent. Morphine is the standard agent to ahich all others compare, and still remains today the standard starting agent for pain control. Long acting formulations of morphine and oxycodone now exist, which has simplified chronic management of both malignant and nonmalignant pain. Treating nonmalignant pain with narcotics has become less controversial than in the past, but rigorous screening and follow-up of patients selected for this type of therapy is strongly recommended.

Mechanism of action

Opiates work by stimulating opiate receptors in the spinal cord and CNS, and possibly in the periphery. Multiple subtypes have now been identified, with some receptors more involved in analgesia, and others with side effects. Different agents have been shown to stimulate these receptors in varying degrees. The agents are classified as naturally occuring opium alkaloids, such as morphine, codeine, and papaverine, semisynthetic derivatives, such as oxycodone, hydromorphone, and heroin, and the synthetic opiods, such as fentanyl, meperidine, and methadone.

Side effects of narcotic are varied, with degree of side effects the limiting factor in therapy. Narcotics alone do not have any toxic dose which must not be surpassed. If one agent is not effective at a particular dose, the solution is to increase the doseage until analgesia is achieved or side effects are encountered. The side effect profile also differs with the route of administration; thus, if a patient fails oral narcotics secondary to intolerable side effects, they may achieve effective pain relief by administering the same agent intravenously, or even epidurally or intrathecal. According to the World Health Organization Cancer Pain Treatment algorithm, roughly 90% of patients should be able to be treated with oral narcotics, with or without adjuvant agents. Side effects commonly seen are sedation, respiratory depression, constipation, confusion, nausea, and pruritus (particularly with epidural agents).

Intrathecal agents, currently morphine, fentanyl, sufentanil, dilaudid, and meperidine, are delivered directly to the main site of action, the spinal cord. As such, a much lower dose is needed to achieve the same degree of analgesia. A standard conversion factor from oral to intrathecal is 300 to 1. Thus, if the patient was on 120 mg of MS Contin TID., he or she might only require 1 mg/day of intrathecal morphine. One can see how dramatic a difference this would make on constipation, for example.

The different agents used in our clinical practice are listed below, with their respective potency.

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