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Background Information
Opioids are the major class of analgesics used in the management of moderate to severe pain because of their effectiveness, ease of titration, and favorable risk-to-benefit ratio. Opioids produce analgesia by binding to specific receptors both within and outside the CNS.438; 197 Opioid analgesics are classified as full agonists, partial agonists, or mixed agonist-antagonists, depending on the specific receptors to which they bind and their intrinsic activity at that receptor.
Commonly used full agonists include morphine, hydromorphone, codeine, oxycodone, hydrocodone, methadone, levorphanol, and fentanyl. These opioids are classified as full agonists because they do not have a ceiling to their analgesic efficacy and will not reverse or antagonize the effects of other opioids within this class given simultaneously. Side effects include constipation, nausea, urinary retention, confusion, sedation, and respiratory depression.
Buprenorphine is a partial agonist. It has a relatively low intrinsic efficacy at the opioid receptor in comparison to full opioid agonists and displays a ceiling effect to analgesia.
Mixed agonist-antagonists in clinical use include pentazocine, butorphanol tartrate, dezocine, and nalbuphine hydrochloride. These drugs have an analgesic ceiling. In contrast to full agonists, these drugs block opioid analgesia at one type of opioid receptor (mu) or are neutral at this receptor while simultaneously activating a different opioid receptor (kappa). Patients receiving full opioid agonists should not be given a mixed agonist-antagonist because doing so may precipitate a withdrawal syndrome and increase pain.
Morphine is the most commonly used opioid for moderate to severe pain because of its availability in a wide variety of dosage forms, its well-characterized pharmacokinetics and pharmacodynamics, and its relatively low cost.
Meperidine may be useful for brief courses (e.g., a few days) to treat acute pain and to manage rigors (shivering) induced by medication, but it generally should be avoided in patients with cancer because of its short duration of action (2.5 to 3.5 hours) and its toxic metabolise, normeperidine. This metabolize accumulates, particularly when renal function is impaired, and causes CNS stimulation, which may lead to dysphoria, agitation, and seizures.242 Therefore, meperidine should not be used if continued opioid use is anticipated.
Equianalgesic doses of commonly used opioids and starting doses for those drugs are listed in Table 10 and Table 11.
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