Opioid agonists, mixed agonist-antagonists and partial agonists

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Opioid agonists, mixed agonist-antagonists and partial agonists

Prerequisite basic sciences

Prerequisite basic sciences

Prerequisite basic sciences


Opioid agonists, mixed agonist-antagonists and partial agonists


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USMLE® Step 1 questions

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USMLE® Step 2 questions

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Opioid agonists, mixed agonist-antagonists and partial agonists

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USMLE® Step 1 style questions USMLE

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USMLE® Step 2 style questions USMLE

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A 44-year-old woman presents to the emergency department following a first-time seizure. The patient was recently seen in the emergency department for symptomatic nephrolithiasis and was discharged with pain medications and urology follow-up. Her roommate, who is at the bedside, does not recall which medication she was prescribed. Past medical history includes depression, chronic pain, anxiety, and hypertension. Her current medications include sertraline, venlafaxine, amlodipine, and alprazolam. Temperature is 37.3°C (99.2°F), pulse is 111/min, respirations are 22/min, and blood pressure is 163/105 mmHg. Physical examination reveals a confused postictal individual with diaphoresis and mydriasis. Clonus and hyperreflexia are elicited in the lower extremities bilaterally. Which of the following medications is most likely responsible for this patient’s clinical findings?  

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Acetylcholine (ACh)

opioid analgesics p. 572

Acute pulmonary edema

opiod analgesics p. 572

Adverse effects/events

opioid analgesics p. 572

Calcium channels

opioid effect on p. 572


opioids for p. 572

opioid withdrawal p. 594


opioid effects p. 572


opioid effects p. 572


opioids for withdrawal p. 572


opioids p. 573


for opioid toxicity p. 249, 572, 594


opioid toxicity p. 572, 594

Norepinephrine (NE)

opioid effect on p. 572

Opioids p. 573

Beers criteria p. 248

intoxication and withdrawal p. 594

pentazocine and p. 573

sleep apnea p. 703

toxicity treatment p. 249

Potassium channels

opioid effect p. 572

Pregnancy p. 657

opiate use during p. 639

Pulmonary edema

opioids for p. 572

Respiratory depression

opioids p. 573


with opioid analgesics p. 572


Opioid agonists are medications used mainly to control acute or chronic pain in particular situations.

Some of them are also used to treat diarrhea and cough. When treating pain, the goal should be to use short-acting opioids at the lowest effective dose for just a few days, and slowly increase their dose only as needed.

As a class, opioids share one thing in common, they bind to opioid receptors in the brain, spinal cord, and gastrointestinal tract.

Some are endogenous, meaning they are produced naturally by the body, like endorphins, short for endogenous morphine.

But others are exogenous, meaning they come from outside the body, like heroin and morphine, which come from the opium poppy; a flowering plant that oozes a milky white liquid.

To understand how opioids work, let’s zoom into a region of the brain tissue that has opioid receptors.

Normally, in the absence of endorphins, inhibitory neurons secrete a neurotransmitter called gamma-aminobutyric acid, or GABA, that prevents nearby neurons from releasing neurotransmitters like dopamine, serotonin, and norepinephrine.

Now, let’s say someone goes to play a rigorous game of badminton. Exercise releases endorphins which activate the three major opioid receptors located on the inhibitory neurons, called the mu, kappa, and delta receptors.

As endorphins bind to these receptors, they block the inhibitory neuron from releasing GABA, allowing the dopamine, serotonin, and norepinephrine secreting neurons to freely unload their neurotransmitters, which then get picked up by another neuron in the same area.

Norepinephrine and serotonin release takes place in pain processing regions of the brain like the thalamus, brainstem, and spinal cord, resulting in a decreased sensitivity to pain.


Opioid full agonists are drugs that bind to and activate opioid receptors in the body. They are used to treat pain and can also produce feelings of euphoria, which has led to their abuse and addiction potential. Examples of opioid agonists include morphine, codeine, and oxycodone.

Mixed agonist-antagonists bind to and activate opioid receptors to a certain extent, but also have the ability to block or inhibit the effects of other opioids. They can also be used to treat pain and may have a lower risk of abuse and addiction compared to full agonists.


  1. "Katzung & Trevor's Pharmacology Examination and Board Review,12th Edition" McGraw-Hill Education / Medical (2018)
  2. "Rang and Dale's Pharmacology" Elsevier (2019)
  3. "Behavioral Effects of Opioid Full and Partial Agonists During Chronic Buprenorphine Treatment" Journal of Pharmacology and Experimental Therapeutics (2019)
  4. "Opioid Use Disorder: Medical Treatment Options" Am Fam Physician (2019)
  5. "Primary care management of opioid use disorders: Abstinence, methadone, or buprenorphine-naloxone?" Can Fam Physician (2017)
  6. "Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th Edition" McGraw-Hill Education / Medical (2017)

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