Opioids
Reviewed and revised 26 August 2014
OVERVIEW
- An opioid is any psychoactive chemical that resembles opiates in their pharmacological effects by binding opioid receptors.
- They may be endogenous or exogenous compounds, and may be naturally occurring or synthetic
CLASSIFICATION OF OPIOIDS
Opioid analgesics may be:
- Pure agonists of specific opioid receptors (notably the mu receptor)
- Mixed agonist – antagonist drugs with opposing effects at distinct receptor subtypes
- Partial mu agonists
The affinity of individual opioid analgesics for receptors varies
- No ceiling effect to analgesia is found with the pure agonists, in contrast to the partial agonist and mixed agonist – antagonist opioids, which demonstrate a ceiling response above which an increase in dose does not produce any additional increase in effect.
OPIOID RECEPTORS AND EFFECTS
Opioid receptors are distributed widely in the:
- Brain (supraspinal sites)
- Spinal cord
- Digestive tract (peripheral sites)
The three principle opioid receptors are:
| 1 | 2 | 3 | 4 |
|---|---|---|---|
| Receptor | Opioid class | Location | Possible Functions |
| Mu subtypes include: μ1, μ2, μ3 | Endorphins |
| Analgesia/ physical dependence/ respiratory depression/ miosis/ Euphoria/ reduced GIT motility/ physical dependence / Possible vasodilation |
Kappa subtypes include: κ1, κ2, κ3 | Dynorphins |
| Analgesia/ convulsant effects/ dysphoria/ Respiratory depression/ reduced GIT motility |
Delta subtypes include: δ1, δ2 | Enkephalins |
| Analgesia, (less than mu) |
Mechanisms of opioid analgesia (primarily via mu receptors, though delta and kappa receptors also mediate analgesic effects)
- Presynaptic inhibition of neurotransmitter release from C-fiber terminals
- Postsynaptic inhibition of evoked activity in nociceptive pathways
- Disinhibition of other circuits regulating nociceptive transmission
- Supraspinal opioids increase descending inhibition of spinal nociceptive transmission
ENDOGENOUS OPIOIDS
The endogenous opioids include:
- Dynorphins
- Enkephalins
- Endorphins
- Endomorphins
- Nociceptin
OPIOID MEDICATIONS
- Natural opiates (alkaloids contained in the resin of the opium poppy) e.g. morphine, codeine
- Esters of morphine opiates e.g. diacetylmorphine (heroin)
- Semi-synthetic opioids (created from natural opiates or morphine esters) e.g. hydromorphone, oxycodone, and buprenorphine
- Synthetic opioids e.g. fentanyl, alfentanil, remifentanil, pethidine, levorphanol, methadone, tramadol and dextropropoxyphene
References and Links
CCC Pharmacology Series
Respiratory: Bosentan, Delivery of B2 Agonists in Intubated Patients, Nitric Oxide, Oxygen, Prostacyclin, Sildenafil
Cardiovascular: Adenosine, Adrenaline (Epinephrine), Amiodarone, Classification of Vasoactive drugs, Clevidipine, Digoxin, Dobutamine, Dopamine, Levosimendan, Levosimendan vs Dobutamine, Milrinone, Noradrenaline, Phenylephrine, Sodium Nitroprusside (SNiP), Sotalol, Vasopressin
Neurological: Dexmedetomidine, Ketamine, Levetiracetam, Lignocaine, Lithium, Midazolam, Physostigmine, Propofol, Sodium Valproate, Sugammadex, Thiopentone
Endocrine: Desmopressin, Glucagon Therapy, Medications and Thyroid Function
Gastrointestinal: Octreotide, Omeprazole, Ranitidine, Sucralfate, Terlipressin
Genitourinary: Furosemide, Mannitol, Spironolactone
Haematological: Activated Protein C, Alteplase, Aprotinin, Aspirin, Clopidogrel, Dipyridamole, DOACs, Factor VIIa, Heparin, LMW Heparin, Protamine, Prothrombinex, Tenecteplase, Tirofiban, Tranexamic Acid (TXA), Warfarin
Antimicrobial: Antimicrobial Dosing and Kill Characteristics, Benzylpenicillin, Ceftriaxone, Ciprofloxacin, Co-trimoxazole / Bactrim, Fluconazole, Gentamicin, Imipenem, Linezolid, Meropenem, Piperacillin-Tazobactam, Rifampicin, Vancomycin
Analgesic: Alfentanil, Celecoxib, COX II Inhibitors, Ketamine, Lignocaine, Morphine, NSAIDs, Opioids, Paracetamol (Acetaminophen), Paracetamol in Critical Illness, Tramadol
Miscellaneous: Activated Charcoal, Adverse Drug Reactions, Alkali Therapies, Drug Absorption in Critical Illness, Drug Infusion Doses, Epidural Complications, Epidural vs Opioids in Rib Fractures, Magnesium, Methylene Blue, Pharmacology and Critical Illness, PK and Obesity, PK and ECMO, Sodium Bicarbonate Use, Statins in Critical Illness, Therapeutic Drug Monitoring, Weights in Pharmacology
Toxicology: Digibind, Flumazenil, Glucagon Therapy, Intralipid, N-Acetylcysteine, Naloxone, Propofol Infusion Syndrome
Critical Care
Compendium
Chris is an Intensivist and ECMO specialist at The Alfred ICU, where he is Deputy Director (Education). He is a Clinical Adjunct Associate Professor at Monash University, the Lead for the Clinician Educator Incubator programme, and a CICM First Part Examiner.
He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives. He was one of the founders of the FOAM movement (Free Open-Access Medical education) has been recognised for his contributions to education with awards from ANZICS, ANZAHPE, and ACEM.
His one great achievement is being the father of three amazing children.
On Bluesky, he is @precordialthump.bsky.social and on the site that Elon has screwed up, he is @precordialthump.
| INTENSIVE | RAGE | Resuscitology | SMACC