Author: Dr Shirish P Amatya
Introduction: Why Pharmacotherapy Matters in Pain Medicine
Pain is not a single disease—it is a clinical outcome driven by different mechanisms. Pharmacotherapy remains a core pillar of pain management because it is:
- Easily self-administered
- Often rapid in onset and can be long-acting
- Intended to have minimal side effects (when chosen appropriately)
- Expected to effectively control pain
- Should act on the pathological part of the pain pathway
- Must be safe and combinable with other therapies and in patients with comorbidities
Pain Mechanisms: Nociceptive and Neuropathic Pain
Broadly, pharmacotherapy is selected based on whether pain is predominantly:
- Nociceptive pain (tissue injury/inflammation-driven)
- Neuropathic pain (lesion/disease of the somatosensory system)
Most real-world patients have mixed pain, so treatment often requires analgesics + adjuvants.
Core Analgesics
1) NSAIDs (Non-Steroidal Anti-Inflammatory Drugs)
Common NSAIDs (Non-selective)
- Aspirin (acetylsalicylic acid)
- Ibuprofen
- Indomethacin
- Aceclofenac
- Diclofenac (relatively more COX-2 activity)
- Etodolac
- Ketorolac
- Mefenamic acid
COX-2 Selective NSAIDs (Coxibs)
- Celecoxib
- Etoricoxib
Why NSAIDs Work (and why they are limited)
NSAIDs are effective analgesics, but their clinical utility is limited by dose-related adverse effects and toxicity:
- Non-selective NSAIDs: higher risk of GI and renal adverse effects
- COX-2 inhibitors: relatively less GI toxicity, but associated with cardiovascular and renal complications
Important safety note (celecoxib): clinical experience has been positive, but concerns about stroke/cardiac risk led to an FDA “black box” warning.
NSAIDs vs COX-2 Inhibitors: Practical Decision Points
Use in mild to moderate pain, with the following rules:
- Use the lowest effective dose for the shortest possible duration
- Use caution in elderly and renal impairment
- Both target COX-2 induced by injury/inflammation
- GI side effects are generally less with coxibs
- Coxibs require caution in active cardiovascular disease or thrombo-embolic history
- Coxibs can be used in aspirin-sensitive asthma
- Coxibs have no effect on platelet aggregation
Clinical takeaway: NSAIDs are valuable for inflammatory nociceptive pain, but risk stratification (GI/renal/CV) is non-negotiable.
2) Paracetamol (Acetaminophen)
Paracetamol has been used for more than a century and remains a first-line option for mild to moderate pain and fever. pharmacotherpy of pain manageme…
Proposed Mechanisms (multimodal)
- Central action on hypothalamic heat regulation
- Inhibits central prostaglandin synthesis (COX-3 concept noted)
- Peripheral COX inhibition (limited)
- Activates serotonergic antinociceptive pathways
- Modulates endogenous cannabinoid system
Clinical profile
- Minimal anti-inflammatory action
- No antiplatelet activity (noted as useful post-prolotherapy pain context)
- Lower GI irritation potential than NSAIDs
- Major risk: dose-dependent hepatotoxicity
Typical dosing
- 500–1000 mg every 6–8 hours
- Maximum < 4 g/day
When paracetamol is especially useful
Considered the drug of choice when NSAIDs are unsuitable (examples listed):
- Bronchial asthma
- Peptic ulcer disease
- Hemophilia
- Salicylate sensitivity
- Children under 12 years
- Pregnancy or breastfeeding
Opioid Analgesics (Opioid Therapy in Pain Management)
Opioid Classification (by origin)
- Natural: morphine, codeine
- Semi-synthetic: oxycodone, diacetylmorphine (heroin), hydromorphone, buprenorphine (partial agonist), naloxone (antagonist)
- Synthetic: tramadol, tapentadol, pethidine, fentanyl, alfentanil, sufentanil, remifentanil, methadone, pentazocine, nalbuphine
Opioid Classification (by receptor action)
- Pure agonists: morphine, pethidine, codeine, fentanyl, methadone
- Partial agonist: buprenorphine
- Agonist–antagonists: nalbuphine, pentazocine
- Antagonists: naloxone, naltrexone
Opioid Receptors
- μ (mu), κ (kappa), δ (delta)
Routes of Administration
- Oral, rectal (absorption similar to oral)
- IV, IM, SC
- Intranasal/inhalation
- Transdermal (notably fentanyl kinetics)
- Neuraxial: epidural/intrathecal (must be preservative-free)
Key Opioids: Practical Notes
Morphine
- Most widely used opioid; can be given by all routes
- Oral bioavailability: 30–35%
- Intrathecal dose is ~300× less than oral (dose equivalence concept provided)
Metabolism and caution
- Conjugation to morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G)
- Excreted mainly by kidneys
- Use caution in hepatic impairment, renal impairment, and elderly
Codeine
- Oral tablet: 15 mg, 30 mg
- Metabolized to morphine
- Example equivalence noted: 60 mg codeine ≈ 650 mg aspirin (as presented)
- Also used as antitussive; often in cough mixtures
- Typically causes minimal sedation, nausea, vomiting, constipation (relative statement)
Fentanyl
- ~100× potency of morphine; highly lipid soluble
- Fast onset: 30–40 seconds; short duration due to redistribution
- IV bolus: 1–2 μg/kg; infusion 0.1–0.5 μg/kg/hr
- Transdermal patch options: 12.5, 25, 50, 75, 100 μg/hr
- Onset: 12–16 hours
- Duration: 72 hours
- Change every 3 days
Tramadol
- Atypical opioid with dual mechanism
- Opioid receptor binding
- Inhibits serotonin & noradrenaline reuptake (descending inhibition)
- Forms: oral 50 mg, combo with paracetamol (37.5 mg noted), injection 50 mg/ml
- Max dose: 400 mg/day
- Hepatic metabolism (active metabolite O-desmethyltramadol), renal excretion
- Reduce daily dose in chronic renal failure
Buprenorphine
- Highly lipophilic; partial μ agonist, κ antagonist
- 25–50× more potent than morphine (as presented)
- Example equivalence: 0.3 mg oral = 10 mg morphine
- Onset: ~30 minutes; duration: 6–9 hours
- Sublingual: 0.2 mg TDS
- Transdermal patch: 5/10/20 mcg/hr, effective 7 days
Opioid-Related Adverse Effects (High-Yield)
- GI: nausea, vomiting, constipation
- Sedation
- Respiratory depression (especially overdose)
- Pruritus
- Tolerance
- Physical dependence and addiction risk
Most adverse effects may subside after a few days except constipation, which commonly persists.
Adjuvants / Co-Analgesics (Neuropathic Pain and Mixed Pain)
1) Anticonvulsants for Pain (Commonly used in neuropathic pain)
1st Generation (listed)
Carbamazepine, ethosuximide, phenobarbital, phenytoin, primidone, valproic acid etc
2nd Generation (listed)
Felbamate, gabapentin, pregabalin, lacosamide, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, vigabatrin, zonisamide etc
Clinical positioning: In practice, anticonvulsants are foundational “neuropathic pain medications” and are often combined with antidepressants when indicated.
2) Antidepressants for Pain
Tricyclic Antidepressants (TCAs)
Mechanism: block reuptake of serotonin and norepinephrine.
Key practical points:
- Clinically demonstrable improvement: 2–3 weeks
- Long elimination half-life: 1–4 days
- Side effects: urinary retention, orthostatic hypotension, decreased gastric acid
Examples: amitriptyline, nortriptyline, desipramine (noted as better in elderly)
SSRIs
- Fewer side effects relative to TCAs (as presented)
- Option for patients who cannot tolerate TCAs
Mechanism described: block serotonin reuptake via sodium/potassium ATPase pump concept (as presented)
Examples: fluoxetine, sertraline, escitalopram (noted for MDD, PTSD, anxiety)
SNRIs
- Duloxetine and venlafaxine noted as effective for neuropathic pain
3) Muscle Relaxants (Mechanism-based grouping)
- Spinal cord/subcortical inhibition of polysynaptic reflex arc: chlorzoxazone, methocarbamol
- Brain stem/locus coeruleus: cyclobenzaprine
- Central alpha-2 agonists: tizanidine
- GABA-B agonists: baclofen; benzodiazepines
- Thiocolchicoside described with receptor actions and seizure risk note (as presented)
- Tolperisone: piperidine derivative; central Na/Ca channel blockade
Anti-Inflammatory and Interventional Pharmacology Adjuncts
Corticosteroids (Pain practice relevance)
Key mechanisms:
- Reduce inflammation by inhibiting phospholipase A2
- Membrane stabilizing effects; blocks nociceptive C-fiber input and suppresses ectopic discharge
Common agents listed:
- Triamcinolone
- Methylprednisolone (40 mg/ml; particulate/depo-medrol mentioned)
- Betamethasone
- Dexamethasone
- Deflazacort pharmacotherpy of pain manageme…
Dose limit concept provided (depot steroids):
- 3 mg/kg/year
- 6 mg/kg/lifetime
NMDA Receptor Antagonist: Ketamine in Chronic Pain
Ketamine: Mechanism and Routes
- Phencyclidine derivative
- Primary mechanism: non-competitive NMDA receptor antagonist
Routes: - IV
- IM/SC (onset 10–15 min)
- Oral (bioavailability ~20%) / sublingual (~30%) / rectal (~30%)
Metabolized in liver; metabolites ~1/5 as potent as ketamine (as presented)
Analgesic Uses of Ketamine (as listed)
- IV infusions for chronic pain (e.g., fibromyalgia)
- Low dose infusion: 0.1–0.5 mg/kg/hr can provide excellent analgesia
- Reduces postoperative opioid requirements
- Neuropathic pain including CRPS
- Adjunct for cancer pain, especially neuropathic component
Local Anesthetics and Other Modalities
Local Anesthetics (Lignocaine/Lidocaine)
- Systemic lignocaine noted for “central desensitization”
- Mechanism: blocks sodium channels → prevents depolarization and impulse transmission
Other items mentioned
- Botulinum toxin
- Transdermal patches (category closing note)
Clinical Approach: How to Choose Pain Medications (Practical Framework)
Use a mechanism-based and risk-stratified approach:
- Identify pain mechanism: nociceptive vs neuropathic vs mixed
- Start with safest effective option for the patient’s comorbidity profile
- Limit NSAID exposure (dose/duration) and assess GI/renal/CV risk
- Use adjuvants early when neuropathic features dominate
- Reserve opioids for appropriate indications, with side-effect planning (constipation prophylaxis is essential)
- Consider advanced options (e.g., ketamine infusion) for selected refractory neuropathic/nociplastic-like syndromes, where appropriate
Frequently Asked Questions on Pharmacotherapy
What is the goal of pharmacotherapy in pain management?
To control pain effectively while targeting the pathological component of the pain pathway, with minimal side effects and safe combination with other therapies and comorbid disease management.
What is the difference between non-selective NSAIDs and COX-2 inhibitors?
Non-selective NSAIDs have higher GI and renal adverse effects. COX-2 inhibitors generally reduce GI risk but may increase cardiovascular and renal complications and should be used cautiously in thrombo-embolic disease.
When is paracetamol preferred over NSAIDs?
When NSAIDs are contraindicated or poorly tolerated (e.g., asthma, peptic ulcer disease, salicylate sensitivity, pregnancy/breastfeeding, bleeding disorders). Paracetamol has less GI irritation but can cause dose-dependent hepatotoxicity.
Which opioid is commonly used as a transdermal patch?
Fentanyl and buprenorphine are listed with transdermal patch dosing/kinetics (fentanyl patches typically changed every 72 hours; buprenorphine patches effective for 7 days, as presented).
What are the most persistent opioid side effects?
Many effects reduce after a few days, but constipation typically persists and must be proactively managed.
Why are antidepressants and anticonvulsants used in pain medicine?
They are used as adjuvant analgesics (co-analgesics), particularly relevant to neuropathic pain mechanisms and descending inhibitory pathway modulation (as presented in drug mechanisms/classes).
What is the role of ketamine in chronic pain?
Ketamine (NMDA antagonist) may be used via low-dose infusion for chronic pain conditions including fibromyalgia and neuropathic pain such as CRPS, and as adjunct in cancer pain with neuropathic components.
