Analgesics

Learning Objectives

At the end of this session, you should be able to:

• Understand the classification of analgesics
  
  

• Explain the mechanisms of action
  
  

• Know the clinical uses, side effects, and correct dosages of commonly used analgesics
  
  

• Understand the role of multimodal analgesia
  
  

Definition of Analgesia

Analgesia is the relief of pain without loss of consciousness. It can be:

• Central (acting in the brain/spinal cord)
  
  

• Peripheral (acting at the site of injury or in the nerves)
  
  

Classification of Analgesics

1. Non-Opioid Analgesics

• Paracetamol
  
  

• Non-steroidal anti-inflammatory drugs (NSAIDs): ibuprofen, diclofenac, ketorolac
  
  

2. Opioid Analgesics

• Weak opioids: codeine, tramadol
  
  

• Strong opioids: morphine, fentanyl, pethidine
  
  

3. Adjuvant Analgesics

• Ketamine (at sub-anaesthetic doses)
  
  

• Gabapentin, pregabalin (for neuropathic pain)
  
  

• Antidepressants (e.g., amitriptyline)
  
  

4. Local Anaesthetics

• Lignocaine
  
  

• Bupivacaine (used in infiltration, blocks, epidurals)
  
  

Paracetamol (Acetaminophen)

• Centrally acting, inhibits prostaglandin synthesis in the CNS
  
  

• Weak anti-inflammatory activity
  
  

• Excellent safety profile if not overdosed
  
  

Dosage:

• Adults: 1 g PO/IV every 6 hours (max 4 g/day)
  
  

• Children: 15 mg/kg every 6 hours
  
  

Side Effects:

• Generally well tolerated
  
  

• Hepatotoxicity in overdose (liver failure risk at >7.5 g in adults)
  
  

Antidote in overdose:

• N-acetylcysteine (NAC)
  
  

NSAIDs (Non-Steroidal Anti-Inflammatory Drugs)

Examples:

• Ibuprofen, Diclofenac, Ketorolac, Aspirin, Indomethacin
  
  

Mechanism:

• Inhibit cyclo-oxygenase (COX-1 and COX-2) enzymes
  
  

• ↓ Prostaglandins → ↓ inflammation, pain, and fever
  
  

Dosage:

• Ibuprofen: 400–600 mg PO every 6–8 hours (max 2.4 g/day)
  
  

• Diclofenac: 50 mg PO every 8 hours or 75 mg IM
  
  

• Ketorolac: 30 mg IV/IM every 6 hours (max 120 mg/day; limit use to ≤5 days)
  
  

Side Effects:

• Gastritis, GI bleeding
  
  

• Renal impairment
  
  

• Bronchospasm (esp. in asthmatics)
  
  

• Platelet dysfunction
  
  

Contraindications:

• Peptic ulcer disease
  
  

• Renal failure
  
  

• Bleeding disorders
  
  

• Asthma (with caution)
  
  

• Third trimester of pregnancy
  
  

Opioid Analgesics

Opioids bind to mu (μ), delta (δ), and kappa (κ) receptors in the CNS to inhibit pain perception.

Weak Opioids

• Codeine: 30–60 mg PO every 4–6 hours (ceiling effect at 240 mg/day)
  
  

• Tramadol: 50–100 mg PO/IV every 6–8 hours (max 400 mg/day)
  
  

  • Also inhibits serotonin and norepinephrine reuptake
    
    

Strong Opioids

Morphine

• Prototype opioid
  
  

• For moderate to severe pain
  
  

• Metabolised in the liver, excreted by kidneys
  
  

Dosage:

• IV: 2–10 mg every 2–4 hours
  
  

• SC/IM: 5–15 mg every 4 hours
  
  

• PO (slow release): 10–30 mg every 12 hours

Side Effects:

• Respiratory depression
  
  

• Sedation
  
  

• Nausea and vomiting
  
  

• Constipation
  
  

• Urinary retention
  
  

• Pruritus
  
  

• Hypotension (especially IV)
  
  

Antidote:

• Naloxone (opioid antagonist): 0.4–2 mg IV every 2–3 minutes PRN, up to 10 mg
  
  

Fentanyl

• Synthetic opioid, 100 times more potent than morphine
  
  

• Very lipid-soluble → rapid onset and short duration
  
  

Dosage:

• IV: 25–100 mcg every 30–60 minutes
  
  

• Also available as transdermal patch (not for acute use)
  
  

Uses:

• Intraoperative analgesia
  
  

• Postoperative pain
  
  

• Sedation in ICU
  
  

Advantages:

• Minimal histamine release
  
  

• Better cardiovascular stability than morphine
  
  

Disadvantages:

• Respiratory depression
  
  

• Short duration unless given by infusion or patch
  
  

Pethidine (Meperidine)

• Not preferred anymore due to neurotoxicity
  
  

• Metabolite: norpethidine → seizures in renal failure
  
  

Dosage:

• 50–100 mg IM/IV every 4 hours
  
  

Side Effects:

• Seizures
  
  

• Less constipation than morphine
  
  

Multimodal Analgesia

Definition:
Combining analgesics from different classes to improve pain control and reduce opioid use.

Example Combination:

• Paracetamol + NSAID + Opioid (as needed)
  
  

• +/- regional block or spinal analgesia
  
  

Benefits:

• Synergistic pain control
  
  

• Lower opioid doses needed
  
  

• Fewer side effects (e.g., less respiratory depression)