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Updated: Sep 30 2019


  • A 29-year-old woman is brought into the emergency department by the police due to being found unresponsive in the park. At the scene, an empty syringe was found. Her respirations are 6/min. Physical exam is significant for depressed mental status, bilateral track marks, and miotic pupils.
  • Opioids act on mu (most important), kappa, and delta transmembrane receptors in the central and peripheral nervous system
    • mu, kappa, and delta receptors are Gi protein coupled
      • inhibition of adenylyl cyclase → decrease cAMP concentration → increased K+ conductance + decreased Ca2+ conductance 
        • inhibits presynaptic neurotransmitter release
          • prevents release of acetylcholine, norepinephrine, serotonin, glutamate, and substance P
        • inhibits postsynaptic depolarization
  • Endorphines, enkephalins, and dynorphins are examples of endogenous opioid peptides
  • Pharmacokinetics
    • morphine, hydromorphone, and oxymorphone undergo first pass metabolism
      • morphine gets metabolized to morphine-6-glucoronide, which is highly active (the active analgesic)
    • opioids are generally metabolized by liver into inactive glucuronide conjugates
      • after metabolism they get excreted by the kidneys
    • codeine, oxycodone, and hydrocodone are metabolized by hepatic cytochrome enzymes
      • cytochromes have genetic variability → variable analgesic response
    • meperidine can get metabolized to normeperidine N-demethylation
      • normeperidine can lead to anxiety, tremors, and seizures
    • heroin gets metabolized into 6-monoacetylmorphine, which can be detected on urine screening
Clinical Use
Drug Classification
Subclass Medication


  • Morphine
    • pain management (acute) 
  • Methadone
    • detoxification and maintenance (agonist) treatment of opioid addiction 
    • pain management
  • Codeine
    • pain management

Mixed agonist-antagonist

  • Buprenorphine
    • maintenance treatment of opioid addiction
    • pain management
  • Naltrexone
    • treatment of alcohol dependence
    • opioid overdose
  • Methylnaltrexone
    • opioid-induced constipation
  • Naloxone
    • opioid overdose
      • for reversal of opioid depression
  • Dextromethorphan
  • Codeine
  • Tramadol
    • pain management
  • Butorphanol
    • pain management
  • Loperamide/dyphenoxylate
    • manages diarrhea
Side Effect by System
Opiate Pharmacologic Effects by System
System Side Effect

Central nervous

  • Analgesia
  • Euphoria/dysphoria
  • Miosis
    • tolerance does not develop in miosis
  • Sedation
  • Cough reflex inhibition


  • Vasodilation
  • Hypotension
  • Respiratory depression
  • Constipation
    • tolerance does not develop to constipation
  • Increased tone/pressure in the biliary sphincter
  • Nausea and vomiting
  • Urinary retention
  • May prolong labor
  • Increases antidiuretic hormone (ADH) and prolactin secretion
  • Decrease luteinizing hormone (LH) secretion
  • Pruritus
  • Flushing
  • Acute overdose triad
    • miosis, respiratory depression, and coma
      • respiratory acidosis without metabolic compensation 
      • respiratory depression due to ↓ response to ↑ pCO2
  • Tolerance
    • a decrease in pharmacologic effect after prolonged drug usage
      • secondary to down-regulation of opioid receptors
    • miosis and constipation are not affected by tolerance
    • NMDA receptor antagonists (e.g., ketamine) and delta-receptor antagonists have been reported to prevent opioid tolerance
  • Withdrawal side effects
    • pain originating from the CNS
    • flu-like symptoms
      • lacrimation, sweating, rhinorrhea, and salivation
      • muscle cramps and spasms
  • Drug specific side-effects
    • meperidine (seizures and serotonin syndrome with MAOIs/SSRIs)
    • nalbuphine (hallucinations)
  • Head trauma
    • ↑ pCO2 can cause vasodilation and ↑ intracranial pressure
  • Pulmonary dysfunction
    • patient may not be able to compensate for respiratory depression
  • Liver/kidney dysfunction
    • poor excretion mechanisms may result in accumulation
  • Adrenal/thyroid deficiencies
    • opiates may show greater effects
  • Pregnancy
    • fetus may show dependency
    • exception is meperidine
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