Opioid Analgesics: Pharmacology, Safe Use, Overdose and Dependence

Core Learning Objectives

• Classify psychoactive substances by receptor pharmacology and physiological effects.
• Detail the MOA, pharmacokinetics, clinical indications, and contraindications of major drug classes.
• Recognise predictable adverse effects, toxic toxidromes, and dangerous drug interactions.
• Prioritise emergency assessment using a logical primary survey (ABCDE + glucose/temp).
• Outline evidence-based overdose care, withdrawal management, and formal rehabilitation modalities.

---

Part 1: The Emergency Clinical Workflow

The name of the substance matters, but physiology comes first. A patient with reduced consciousness can deteriorate while staff are still trying to identify what was taken. Approach the patient systematically.

---

Part 2: Detailed Pharmacology by Drug Class

1. Opioid Analgesics

Opioids are essential medicines for surgery, severe trauma, and palliative care, but their identical pharmacology causes sedation, tolerance, and fatal respiratory depression.

Mechanism of Action (MOA)

• Receptor binding: Act primarily as agonists at G-protein-coupled opioid receptors (µ [mu], κ [kappa], and δ [delta]).
  • µ (mu) receptor: Main clinical receptor for analgesia, euphoria, sedation, respiratory depression, miosis, constipation, and physical dependence.
  • κ (kappa) receptor: Contributes to spinal analgesia, sedation, and dysphoria.
  • δ (delta) receptor: Contributes to analgesia and affective modulation.
• Cellular signalling (Gi/o coupled):
  • Presynaptic: Decreases cAMP, closes voltage-gated calcium channels → decreases release of excitatory neurotransmitters (Substance P, glutamate).
  • Postsynaptic: Opens potassium channels → K+ efflux causes hyperpolarisation, decreasing neuronal firing.
• Sites of action: Peripheral tissues, dorsal horn of the spinal cord, brainstem descending pathways, and the thalamus/limbic system (alters emotional response to pain).

Pharmacokinetics (PK) & Classification

Clinical Indications

• Severe acute pain (trauma, burns, post-operative).
• Cancer and palliative pain (including dyspnoea relief in end-of-life care).
• Anesthesia (adjunct to induction/maintenance).
• Opioid Use Disorder (OUD) maintenance (Methadone, Buprenorphine).

Contraindications & Precautions

• Absolute: Acute respiratory depression, severe asthma/COPD without mechanical ventilation, paralytic ileus.
• Relative precautions: Renal impairment (morphine metabolites accumulate, causing neurotoxicity), hepatic failure, raised intracranial pressure (ICP), and sleep-disordered breathing.

Side & Adverse Effects

• Respiratory: Dose-dependent respiratory depression (reduced brainstem sensitivity to CO2).
• GI: Constipation (does not develop tolerance), nausea/vomiting (CTZ stimulation), biliary colic (sphincter of Oddi spasm).
• Neuro: Sedation, miosis (pinpoint pupils), delirium in the elderly.
• Cardiovascular: Hypotension, bradycardia (often due to histamine release with morphine).
• Other: Pruritus (itch), urinary retention, endocrine suppression (hypogonadism), opioid-induced hyperalgesia (paradoxical increased pain).

Drug Interactions

• Fatal Synergy: Opioids + Benzodiazepines + Alcohol = Massive additive respiratory depression.
• Serotonin Syndrome: Tramadol/Pethidine + SSRIs/MAOIs.
• QTc Prolongation: Methadone + Macrolides/Antipsychotics.

Intoxication & Overdose Management

• Classic Toxidrome: Coma + Respiratory Depression (<12 breaths/min) + Miosis (Pinpoint pupils).
• Management: ABCDE. BVM ventilation is the most critical step. Administer Naloxone (titrate to adequate spontaneous breathing, not full wakefulness). Observe for renarcotization because naloxone’s half-life is shorter than most opioids.

Withdrawal & Dependence

• Withdrawal signs: Yawning, lacrimation, rhinorrhoea, piloerection (goosebumps), pupil dilation, severe diarrhoea, bone pain, and anxiety. Intense but rarely fatal.
• Management of OUD: Opioid Agonist Therapy (OAT) with Methadone or Buprenorphine significantly reduces mortality. Avoid abrupt cessation of maintenance treatment. Loss of tolerance post-detoxification heavily increases fatal overdose risk upon relapse.

---

2. Alcohol (Ethanol)

Mechanism of Action (MOA)

• GABA-A Receptors: Positive allosteric modulator (enhances inhibitory GABA transmission).
• NMDA Receptors: Antagonizes excitatory glutamate receptors.
• Reward pathway: Stimulates dopamine and endogenous opioid release in the nucleus accumbens.

Pharmacokinetics

• Metabolism: Follows zero-order kinetics (eliminated at a constant rate regardless of concentration).
• Pathways: Ethanol → (Alcohol Dehydrogenase) → Acetaldehyde (toxic) → (Aldehyde Dehydrogenase) → Acetate.

Contraindications & Precautions

• Hepatic failure, history of pancreatitis, pregnancy (Fetal Alcohol Syndrome), peptic ulcer disease.

Adverse Effects (Chronic)

• Hepatic: Steatosis, alcoholic hepatitis, cirrhosis.
• Neuro: Peripheral neuropathy, cerebellar degeneration, Wernicke-Korsakoff Syndrome (due to severe thiamine/B1 deficiency).
• Cardiovascular: Dilated cardiomyopathy, hypertension.

Drug Interactions

• Disulfiram-like reaction: Metronidazole, Cephalosporins (cause severe vomiting/flushing if alcohol is consumed).
• CNS Depressants: Fatal respiratory depression with opioids/sedatives.
• Paracetamol (Acetaminophen): Chronic alcohol use depletes glutathione and induces CYP2E1, increasing risk of fatal hepatotoxicity even at therapeutic paracetamol doses.

Intoxication Management

• Supportive care, lateral recovery position to prevent aspiration, IV fluids, and strict bedside glucose correction (alcohol inhibits gluconeogenesis).

Withdrawal Management (Medical Emergency)

• Timeline: Tremors/anxiety (6-24h) → Seizures (12-48h) → Delirium Tremens (48-96h; severe confusion, fever, hallucinations, autonomic instability).
• Treatment: Assess with CIWA-Ar scale. Benzodiazepines (Diazepam, Lorazepam) are first-line. Administer IV Thiamine BEFORE Glucose to prevent irreversible Wernicke encephalopathy.
• Relapse Prevention: Acamprosate (modulates glutamate), Naltrexone (blocks reward), Disulfiram (aversive therapy).

---

3. Sedative-Hypnotics (Benzodiazepines, Barbiturates, Z-Drugs, GHB)

Mechanism of Action (MOA)

• Benzodiazepines (BZDs): Bind to allosteric sites on the GABA-A receptor, increasing the frequency of chloride channel opening.
• Barbiturates: Bind to a different site on GABA-A, increasing the duration of chloride channel opening. At high doses, they can directly open the channel without GABA (higher fatality rate).
• GHB (Gamma-hydroxybutyrate): Agonist at specific GHB receptors and weak agonist at GABA-B receptors.

Pharmacokinetics

• Hepatic metabolism. Diazepam and chlordiazepoxide have long-acting active metabolites. Lorazepam, Oxazepam, and Temazepam (LOT) skip phase I CYP metabolism (preferred in liver disease/elderly).

Clinical Indications

• Acute anxiety, status epilepticus, alcohol withdrawal, muscle spasm, procedural sedation.

Contraindications

• Myasthenia gravis, severe sleep apnea, severe respiratory insufficiency.

Adverse Effects

• Anterograde amnesia, daytime somnolence, paradoxical agitation (especially in the elderly/children), ataxia (fall risk).

Intoxication & Overdose Management

• Toxidrome: Slurred speech, ataxia, nystagmus, stupor.
• Management: Supportive airway care. Barbiturate overdose may require urine alkalinization. Flumazenil (BZD antagonist) is rarely used as it precipitates intractable seizures in chronic users.

Withdrawal

• Abrupt cessation causes life-threatening seizures and psychosis. Requires a slow, multi-week medically supervised taper (often using diazepam).

---

4. Stimulants (Cocaine, Amphetamines, Cathinones/Khat)

Mechanism of Action (MOA)

• Cocaine: Blocks the reuptake of Dopamine, Norepinephrine, and Serotonin (DAT, NET, SERT inhibitor). Also acts as a local anesthetic by blocking voltage-gated Na+ channels.
• Amphetamines / Methamphetamine: Enter the neuron via reuptake transporters, force monoamines out of storage vesicles (via VMAT), and reverse the transporter to pump massive amounts of dopamine/norepinephrine into the synapse.

Pharmacokinetics

• Cocaine has a very short half-life (~1 hour); Methamphetamine has a very long half-life (~10-12 hours).

Clinical Indications (Medical)

• ADHD (Methylphenidate, Amphetamine salts), Narcolepsy, severe treatment-resistant depression. Cocaine is used rarely in ENT surgery as a topical vasoconstrictive anesthetic.

Contraindications

• Severe cardiovascular disease, hyperthyroidism, glaucoma, history of psychosis.

Side & Adverse Effects

• Psychiatric: Paranoia, violent agitation, tactile hallucinations (formication / “meth bugs”).
• Physical: Dental decay (“meth mouth” via bruxism/xerostomia), extreme weight loss.

Intoxication & Overdose Management

• Toxidrome: Tachycardia, severe hypertension, mydriasis (dilated pupils), diaphoresis, and life-threatening hyperthermia.
• Complications: Myocardial infarction (coronary vasospasm), aortic dissection, stroke, status epilepticus, rhabdomyolysis.
• Management:
  • Benzodiazepines are the absolute cornerstone for agitation, blood pressure control, and seizure prevention.
  • Aggressive active cooling for hyperthermia.
  • Avoid physical restraints (struggling causes fatal lactic acidosis).
  • Avoid pure beta-blockers (theoretically causes unopposed alpha-vasoconstriction).

Withdrawal (The Crash)

• Profound fatigue, hypersomnia, hyperphagia (massive appetite), severe dysphoria, and high suicidality. Treated with psychiatric support.

---

5. Entactogens / Empathogens (MDMA / Ecstasy)

Mechanism of Action (MOA)

• Primary action is the massive reversal of the Serotonin Transporter (SERT), alongside weaker dopamine/norepinephrine release. Produces euphoria, emotional openness, and energy.

Toxicity & Adverse Effects (Emergency Focus)

• Exercise-Induced Hyponatraemia: MDMA triggers inappropriate ADH secretion. Profuse sweating + drinking massive amounts of free water = dilutional hyponatremia. Causes cerebral oedema, seizures, and death. Treatment: Fluid restriction or Hypertonic Saline.
• Hyperthermia & Rhabdomyolysis: Aggravated by dancing in hot environments. Leads to acute renal failure.
• Serotonin Syndrome: Hyperreflexia, clonus, autonomic instability, fever. Treat with cooling, BZDs, and Cyproheptadine (5-HT antagonist).
• Bruxism: Severe jaw clenching.

---

6. Cannabinoids & Synthetic Cannabinoids

Mechanism of Action (MOA)

• THC: Partial agonist at CB1 (CNS) and CB2 (immune system) G-protein coupled receptors. Inhibits adenylate cyclase.
• Synthetic Cannabinoids (Spice, K2): Full, extremely potent agonists at CB1 receptors.

Clinical Indications (Medical Cannabis/Derivatives)

• Chemotherapy-induced nausea (Dronabinol), multiple sclerosis spasticity (Nabiximols), severe childhood epilepsy (Cannabidiol/CBD), appetite stimulation in HIV/AIDS.

Adverse Effects & Toxicity

• Acute (THC): Tachycardia, conjunctival injection (red eyes), dry mouth, panic attacks, paranoia.
• Chronic (THC): Amotivational syndrome, increased risk of schizophrenia in genetically vulnerable youth.
• Cannabinoid Hyperemesis Syndrome (CHS): Intractable cyclical vomiting in chronic daily users, uniquely relieved by compulsive hot showering. Treated with haloperidol or topical capsaicin to the abdomen.
• Synthetics (Toxicity): Produce violent agitation, profound psychosis, dangerous arrhythmias, acute kidney injury, and status epilepticus. (Not detected on standard drug screens).

---

7. Hallucinogens & Dissociatives

Mechanism of Action (MOA)

• Classic Hallucinogens (LSD, Psilocybin, DMT): Agonists at Serotonin 5-HT2A receptors.
• Dissociatives (PCP, Ketamine, Dextromethorphan): Non-competitive antagonists at NMDA glutamate receptors.

Adverse Effects & Management

• LSD/Psilocybin: Visual illusions, synaesthesia, emotional lability (“bad trips”). Management is a quiet room, a supportive “sitter,” and benzodiazepines if severely agitated.
• PCP (Phencyclidine): Causes extreme violent agitation, superhuman strength (analgesia), and distinctive rotary nystagmus. Requires heavy chemical sedation.
• Ketamine: Chronic abuse destroys the bladder mucosa, causing Ketamine-Induced Ulcerative Cystitis (severe pelvic pain, haematuria, contracted bladder).

---

8. Anticholinergics (Datura, Diphenhydramine Abuse)

Mechanism of Action (MOA)

• Competitive antagonism of muscarinic acetylcholine receptors globally.

Intoxication & Overdose (The Anticholinergic Toxidrome)

• “Mad as a hatter” (agitated delirium, hallucinations)
• “Blind as a bat” (mydriasis, loss of accommodation)
• “Red as a beet” (cutaneous vasodilation/flushing)
• “Hot as a hare” (hyperthermia)
• “Dry as a bone” (anhidrosis/no sweating, dry mucous membranes)
• “Full as a flask” (urinary retention, absent bowel sounds)

Management: Supportive cooling, BZDs for agitation. In severe cases with refractory arrhythmias/delirium, use Physostigmine (an acetylcholinesterase inhibitor) under strict ECG monitoring.

---

9. Inhalants (Solvents, Nitrous Oxide, Nitrites)

Mechanism of Action (MOA)

• Highly lipophilic gases that rapidly cross the blood-brain barrier. MOA varies (often GABA-A enhancement or NMDA antagonism).

Adverse Effects & Toxicity

• Sudden Sniffing Death Syndrome: Solvents (petrol, glue) massively sensitize the myocardium to catecholamines. A sudden scare or running causes a fatal adrenaline surge leading to ventricular fibrillation. (Do not startle these patients!).
• Nitrous Oxide (Whippets): Chronic use irreversibly oxidizes Vitamin B12. Leads to megaloblastic anaemia and Subacute Combined Degeneration of the spinal cord (profound, irreversible peripheral neuropathy and ataxia).
• Amyl Nitrite (Poppers): Induces profound vasodilation (hypotension/syncope) and oxidizes haemoglobin to Methaemoglobinaemia (cyanosis unresponsive to oxygen; blood looks chocolate brown). Treat with Methylene Blue.

---

10. Anabolic-Androgenic Steroids

Mechanism of Action (MOA)

• Agonists at intracellular androgen receptors, altering gene transcription to increase muscle protein synthesis (anabolic) and male sex characteristics (androgenic).

Side & Adverse Effects

• Endocrine: Testicular atrophy and profound hypogonadism upon cessation (due to negative feedback on LH/FSH). Gynecomastia (excess testosterone converts to oestrogen via aromatase). Virilization in females.
• Cardiovascular: Severe dyslipidaemia (↓HDL, ↑LDL), left ventricular hypertrophy, premature MI.
• Hepatic: Hepatotoxicity, peliosis hepatis (blood-filled cysts in the liver).
• Psychiatric: Extreme aggression (“roid rage”), mania, and severe suicidal depression during the withdrawal/crash phase.

---

11. Nicotine and Tobacco

Mechanism of Action (MOA)

• Agonist at central and peripheral Nicotinic Acetylcholine Receptors (nAChRs). Triggers dopamine release in the reward pathways and sympathetic ganglionic stimulation.

Adverse Effects (Chronic)

• Endothelial dysfunction, atherosclerosis, COPD, multiple malignancies (lung, bladder, head/neck).

Withdrawal & Management

• Withdrawal causes intense irritability, cravings, increased appetite, and poor concentration.
• Pharmacotherapy for Cessation:
  • Nicotine Replacement Therapy (NRT): Patches, gum, lozenges.
  • Varenicline: Partial agonist at α4β2 nicotinic receptors (reduces craving and blocks reward of smoking).
  • Bupropion: NDRI antidepressant that lowers cravings.

---

Part 3: Harm Reduction, Rehabilitation & Long-Term Care

Harm Reduction (Clinical Prevention)

Rehabilitation Modalities

Detoxification manages acute physical withdrawal, but it is not a cure. Addiction requires long-term neuro-behavioural rehabilitation.

• Inpatient / Residential Rehab: 30-90 day immersive programs. Ideal for severe polysubstance use, repeated relapses, or lack of safe housing.
• Intensive Outpatient Programs (IOP): Structured therapy for several hours daily, bridging the gap between inpatient care and normal life.
• Therapies Used:
  • Cognitive Behavioural Therapy (CBT): Identifying triggers and maladaptive coping loops.
  • Contingency Management: Providing tangible rewards (vouchers) for negative drug screens (highly effective for stimulant use).
  • Dual Diagnosis Treatment: Simultaneously treating co-occurring PTSD, depression, or bipolar disorder alongside the addiction.
• Peer Support: 12-Step programs (AA/NA) focusing on spiritual surrender, or SMART Recovery focusing on self-empowerment and CBT principles.

---

Part 4: High-Yield Exam & OSCE Scenarios

• Scenario 1: Postoperative patient on PCA pump becomes difficult to rouse.
  Action: Immediately assess breathing. If RR < 10, support ventilation with BVM and administer Naloxone per protocol. Check for co-administered sedatives.
• Scenario 2: Patient on chronic oral morphine for cancer pain has no bowel motion for four days.
  Action: Think opioid-induced constipation (OIC). Exclude malignant bowel obstruction. Do not stop opioids if pain is severe; prescribe a robust stimulant + osmotic bowel regimen (e.g., Senna + Macrogol).
• Scenario 3: Person returns after 60 days in a rehab facility and relapses on heroin.
  Action: High risk of fatal overdose due to rapid loss of tolerance. Educate aggressively, provide naloxone, and reconnect to medication-assisted treatment (OAT) immediately without judgement.
• Scenario 4: Patient taking prescribed Tramadol is started on Fluoxetine (SSRI) and develops agitation, tremor, sweating, and hyperreflexia.
  Action: Diagnose Serotonin Syndrome. Stop both drugs, provide supportive care, BZDs for agitation, and seek emergency review.
• Scenario 5: Young adult at a music festival presents severely agitated, skin is flushed and dry, pupils are fully dilated, HR 130, Temp 39°C.
  Action: Anticholinergic toxidrome (“Dry as a bone, red as a beet…”). Cool the patient, administer BZDs. Avoid haloperidol (can worsen anticholinergic effects).

Knowledge Check

---

Further Study and Clinical References

• Supplied class resource: Pharmacology of Drug Abuse (SlideShare)
• WHO: Psychoactive Drugs
• WHO: Community management of opioid overdose
• UNODC/WHO: International standards for treatment of drug use disorders