Opioid Agonists and Analgesia

• Opioid analgesics include opioid agonists and opioid agonist-antagonists, which are used to treat pain.
• Opioid agonists primarily bind to mu-type opioid receptors, producing analgesic effects.
• Opioid agonist-antagonists bind to both mu and kappa receptors, stimulating and blocking analgesic effects simultaneously, making them less effective than opioid agonists.
• Opioid agonist-antagonists can be used as an alternative to opioids for clients addicted to opioids or for women in labor.

Opioid Agonists

• Opioid agonists are analgesics that are typically administered for moderate to severe pain.
• An additional effect of opioid agonists is that they can also induce sedation and lessen anxiety in preoperative individuals.
• Morphine is the prototype medication for opioid agonists.
• Other medications in the opioid agonists category include fentanyl, meperidine, hydromorphone, and methadone.
• Fentanyl information can be found under general anesthesia in the Neurological System 1 module.
• Meperidine, a synthetic opioid, is typically reserved for clients who cannot tolerate other opioids due to its interactions with other medications.
• Methadone, another synthetic opioid, is primarily used as a substitute for opioids in substance use disorder programs.
• Methadone blocks euphoric effects and reduces cravings in individuals addicted to opioids.
• Other common opioid agonists include codeine, oxycodone, hydrocodone, and tapentadol.

Expected Pharmacologic Action

• Opioid agonists mimic naturally occurring opioids, endorphins, and enkephalins.
• Opioid agonists bind with the mu receptors at opioid receptor sites.
• Stimulation of mu receptors causes analgesia, sedation, euphoria, and respiratory depression.

Adverse Drug Reactions

• Adverse drug reactions, such as respiratory depression and sedation, occur secondary to stimulation of mu opioid receptors.
• Sedation can cause dizziness, lightheadedness, and drowsiness as side effects.
• Opioid agonists decrease intestinal motility, causing constipation.
• Opioid agonists dilate the peripheral vasculature, causing orthostatic hypotension.
• Opioid agonists cause urinary retention by decreasing the client’s perception that the bladder is full.
• Cough suppression is an adverse drug reaction of opioid agonists.
• Opioids have a high potential for misuse due to the euphoria that occurs secondary to stimulation of the mu receptors.
• Tolerance and cross-tolerance with other opioids develops with chronic use, which requires larger doses to achieve the usual effect.

Interventions

• Closely monitor vital signs and oxygen saturation. Auscultate the lungs for congestion.
• Monitor for a decrease in respiratory rate and withhold the medication for rates below 12/min.
• Stimulate breathing and administer an opioid antagonist, such as naloxone, if necessary, to reverse respiratory depression and restore respiratory rate.
• Monitor clients when ambulating in case they experience orthostatic hypotension.
• Monitor bowel function and provide fiber supplements and stool softeners as needed due to risk of constipation.
• Give the medication with food and administer an antiemetic as needed for vomiting.
• Ensure adequate hydration either orally or intravenously.
• Monitor intake and output and watch for manifestations of urinary retention, such as bladder distention.
• Encourage clients to urinate every 4 hours because their perception of the need to urinate diminishes when taking opioid agonists.
• Prepare to insert a urinary catheter to drain the bladder if a client is unable to urinate and bladder distention is evident.
• Encourage clients, especially those who are postoperative, to cough frequently to prevent retention of respiratory secretions.
• Prescribe the lowest possible effective dose on a short-term basis only, to prevent dependence.

Administration

• Obtain baseline vital signs before administration and continue to monitor them throughout therapy.
• Opioid agonists can be given orally, intramuscularly, intravenously, subcutaneously, rectally, or epidurally.
• Make sure clients swallow sustained-release forms whole and do not crush or chew them.
• Dilute opioids when administering them intravenously, as the provider recommends, and give slowly over 4 to 5 minutes.
• Have naloxone and resuscitation equipment readily available.
• Monitor client-controlled analgesia pump use and pump settings carefully.
• For clients receiving opioid agonists for terminal cancer pain, give the medication on a fixed, around-the-clock dosing schedule, rather than as needed.

Safety Alert

• Clients in acute care settings are more often undermedicated for pain than they are overmedicated.
• There is little to no risk of dependence developing on a short-term basis.
• Gradual withdrawal of the medication over a period of several days when the pain resolves will allow the body to adjust and minimize the physical manifestations of withdrawal.
• Opioid withdrawal manifestations include abdominal cramps, diarrhea, agitation, anxiety, hypertension, tachycardia, tremors, muscle pain, pupil dilation, runny nose, and insomnia.
• Opioid use disorder requires a client to have a strong desire for the medication in the absence of pain.
• The amount of pain a client is experiencing is based on what that client says it is, so you should treat it accordingly.

Client Instructions

• Take the medication only when needed and on a short-term basis.
• Do not take an opioid agonist prior to driving or activities requiring mental alertness.
• Sit or lie down immediately if feeling lightheaded.
• Rise slowly from a reclining or sitting position to prevent falls due to dizziness and orthostatic hypotension.
• Increase fluid and fiber intake, as well as activity and exercise, to prevent and/or treat constipation.
• If nausea and vomiting occur after taking an oral form of the medication, take the medication with food or milk.
• Report any difficulty or inability to urinate because of the potential for urinary retention.
• Cough regularly to clear secretions from the throat and chest to prevent pneumonia.

Contraindications and Precautions

• Opioid analgesics are contraindicated in women who are pregnant because morphine is a pregnancy risk medication that can result in respiratory depression of newborn or lead to neonatal opioid withdrawal syndrome.
• Opioid analgesics are contraindicated for clients who have renal failure, increased intracranial pressure, biliary colic, or biliary surgery, as well as clients in preterm labor.
• Use opioid agonists with caution in all clients because it’s a Schedule II controlled substance.
• Administer opioid agonists carefully to older adults and infants, as well as clients with reduced respiratory reserve, head injury, inflammatory bowel disease, prostatic enlargement, hypotension, and hepatic or renal disease.

Interactions

• Opioid agonists interact with CNS depressants (such as barbiturates, phenobarbital, benzodiazepines, and alcohol) by increasing their CNS depressant effects.
• Anticholinergic agents (such as antihistamines and tricyclic antidepressants) can increase anticholinergic effects, causing constipation and urinary retention.
• MAOIs taken along with meperidine can cause hyperpyrexia syndrome, which manifests as excitation, seizures, and a highly elevated temperature.
• Concurrent use of opioid agonists with anti-hypertensives increases their hypotensive effects.
• St. John’s wort may increase sedation.

Question Answer

• A client is about to start taking oral morphine to treat acute pain from an injury. The instructions that you should include are:
  • Take it with food.
  • Rise slowly from sitting or reclining.
  • Increase fluid and fiber intake.
  • Do not take it before driving.