Analgesics: Pain Pathways, Non-Opioid and Opioid Analgesics, Adjuvant Agents, and Pain Management Strategies

Introduction

Analgesics are drugs that relieve pain without loss of consciousness. Pain management involves a multimodal approach combining non-opioid analgesics, opioid analgesics, and adjuvant medications. Effective pain treatment requires understanding pain mechanisms, appropriate drug selection based on pain type and severity, and monitoring for adverse effects and abuse potential.

Pain Physiology

Pain Pathways

Pathway Component	Location	Neurotransmitter	Function
Nociceptors	Peripheral tissues	Substance P, CGRP, glutamate	Detect tissue damage (thermal, mechanical, chemical)
A-delta fibers	Peripheral nerves	Glutamate	Fast, sharp, well-localized pain (first pain)
C fibers	Peripheral nerves	Substance P, CGRP, glutamate	Slow, dull, poorly localized pain (second pain)
Dorsal horn (lamina I, II, V)	Spinal cord	Glutamate, substance P	Synaptic transmission, modulation
Spinothalamic tract	Spinal cord white matter	Glutamate	Ascending projection to thalamus
Thalamus	Midbrain	Glutamate	Relay to cortex
Periaqueductal gray (PAG)	Midbrain	Endogenous opioids	Descending inhibition
Rostral ventromedial medulla (RVM)	Medulla	Serotonin, norepinephrine	Descending modulation (facilitation/inhibition)

Types of Pain

Pain Type	Mechanism	Characteristics	Examples	Responsive To
Nociceptive (somatic)	Activation of nociceptors by tissue damage	Well-localized, aching, sharp, throbbing	Surgical incision, fracture, arthritis	NSAIDs, acetaminophen, opioids
Nociceptive (visceral)	Activation of nociceptors in internal organs	Poorly localized, cramping, pressure, referred	Appendicitis, pancreatitis, labor	Opioids, NSAIDs (less effective)
Neuropathic	Damage/dysfunction of nervous system	Burning, shooting, tingling, electric shock-like	Diabetic neuropathy, postherpetic neuralgia, sciatica	Anticonvulsants, TCAs/SNRIs, topical lidocaine
Nociplastic	Altered nociception without clear tissue/nerve damage	Diffuse, hypersensitivity, fatigue	Fibromyalgia, chronic fatigue syndrome	SNRIs, gabapentinoids, exercise
Inflammatory	Inflammation-mediated sensitization	Redness, heat, swelling, hyperalgesia	Rheumatoid arthritis, infection	NSAIDs, corticosteroids, opioids

Non-Opioid Analgesics

Acetaminophen (Paracetamol)

Aspect	Details
Mechanism	COX inhibition (central), cannabinoid system, serotonergic pathway, TRPV1 modulation (not fully understood)
Analgesic efficacy	Moderate; equivalent to NSAIDs for some pain types
Antipyretic	Yes (via hypothalamic COX inhibition)
Anti-inflammatory	Minimal (peripheral COX inhibition weak)
Onset	30-60 min (oral); 15 min (IV)
Duration	4-6 hours
Adult dose	325-1000 mg q4-6h (max 3000-4000 mg/day)
Pediatric dose	10-15 mg/kg q4-6h (max 75 mg/kg/day)
Metabolism	Hepatic (glucuronidation, sulfation; minor CYP2E1 to NAPQI)
Adverse effects	Hepatotoxicity (overdose), rare skin reactions
Toxicity	>4000 mg/day (adults); N-acetylcysteine antidote
Contraindications	Severe hepatic impairment, severe active liver disease

NSAIDs (Non-Steroidal Anti-Inflammatory Drugs)

Property	Non-Selective NSAIDs	COX-2 Selective
Examples	Ibuprofen, naproxen, diclofenac, ketorolac, indomethacin	Celecoxib, etoricoxib
Mechanism	COX-1 and COX-2 inhibition	Selective COX-2 inhibition
Anti-inflammatory	Good	Good
GI ulcer risk	Moderate-high (COX-1 inhibition)	Lower than non-selective
Bleeding risk	Increased (platelet COX-1 inhibition)	Minimal (no platelet effect)
Renal effects	Reduced GFS, sodium retention	Same (both inhibit renal COX-2)
Cardiovascular risk	Variable	Increased risk with chronic use
Max daily dose (ibuprofen)	3200 mg (adult)	200 mg (celecoxib)
Naproxen	1500 mg/day	N/A

Comparison of Common NSAIDs

Drug	Half-Life	Dosing Frequency	Onset	Relative Potency	Key Notes
Ibuprofen	2 hours	q6-8h	30 min	Moderate	OTC; well-tolerated
Naproxen	12-17 hours	q8-12h	1 hour	Moderate	Higher GI risk than ibuprofen
Diclofenac	2 hours	q8h	30 min	High	Topical form available
Ketorolac	4-6 hours	q6h (max 5 days)	10 min IM	High	Injectable only; short-term use
Indomethacin	4-5 hours	q8-12h	30 min	High	High CNS/GI side effects
Celecoxib	11 hours	q12-24h	1 hour	High	Lower GI risk; sulfonamide allergy

Opioid Analgesics

Opioid Receptor Pharmacology

Receptor	Endogenous Ligand	Agonist Effects	Agonist Examples	Antagonist
Mu-1 (OPRM1)	Beta-endorphin, enkephalins	Supraspinal analgesia, euphoria	Morphine, fentanyl, oxycodone	Naloxone, naltrexone
Mu-2	Same	Respiratory depression, constipation, miosis, physical dependence	Same as mu-1	Same
Kappa (OPRK1)	Dynorphins	Spinal analgesia, dysphoria, psychotomimesis	Pentazocine, nalbuphine	Naloxone (lower affinity)
Delta (OPRD1)	Enkephalins	Spinal analgesia, antidepressant-like	DADLE (experimental)	Naloxone (lower affinity)
ORL1/NOP	Nociceptin/orphanin FQ	Modulation of opioid responses	Nociceptin (experimental)	Various experimental antagonists

Opioid Classification by Efficacy

Category	Examples	Efficacy	Ceiling Effect	Clinical Use
Full mu agonists	Morphine, hydromorphone, fentanyl, oxycodone	High (no ceiling)	No (analgesia continues to increase with dose)	Moderate-severe acute and chronic pain
Partial mu agonists	Buprenorphine	Submaximal (plateau at higher doses)	Yes	Moderate pain, opioid addiction treatment
Mixed agonist-antagonists	Pentazocine, nalbuphine, butorphanol	Variable (kappa agonist, mu antagonist)	Yes	Postoperative pain, labor (butorphanol)
Weak opioids	Codeine, tramadol	Low-moderate	Codeine: prodrug limited by CYP2D6	Mild-moderate pain

Opioid Pharmacokinetics

Drug	Onset (PO)	Duration (PO)	Half-Life	Bioavailability	Metabolism	Active Metabolites
Morphine	30-45 min	4-6 hours	2-4 hours	20-40%	Glucuronidation (UGT2B7)	Morphine-6-glucuronide (active, potent)
Oxycodone	15-30 min	4-6 hours	3-5 hours	60-80%	CYP3A4 (noroxycodone)	Minimal (noroxycodone weak)
Hydromorphone	15-30 min	4-5 hours	2-3 hours	30-50%	Glucuronidation	None (inactive)
Fentanyl	N/A (IV: immediate)	1-2 hours (IV)	3-12 hours (transdermal)	N/A (transdermal: 92%)	CYP3A4	None
Codeine	30-60 min	4-6 hours	2-4 hours	50-90%	CYP2D6 (to morphine)	Morphine (active)
Tramadol	30-60 min	4-6 hours	5-7 hours	70-75%	CYP2D6, CYP3A4	O-desmethyltramadol (M1, active)
Methadone	30-60 min	6-12 hours (analgesia), 24-36h (suppression)	15-30 hours (variable)	40-90%	CYP3A4, CYP2B6	None (but accumulates)

Opioid Adverse Effects

Effect	Incidence	Mechanism	Management
Constipation	40-60%	Mu-2 receptor activation in GI tract (decreased peristalsis, increased sphincter tone)	Stimulant laxatives, stool softeners, increased fluids, fiber
Nausea/vomiting	20-40% (transient)	Chemoreceptor trigger zone mu activation	Antiemetics (ondansetron, metoclopramide), tolerance develops
Respiratory depression	Dose-dependent (5% at standard doses)	Mu-2 inhibition of brainstem respiratory centers	Naloxone, careful titration, monitoring
Sedation	20-60%	Central mu activation	Tolerance develops, consider stimulants if persistent
Pruritus	2-10%	Histamine release (morphine), central mu activation	Antihistamines, nalbuphine, opioid rotation
Urinary retention	1-5%	Increased detrusor tone, sphincter contraction	Monitoring, catheter if needed
Tolerance	Universal with chronic use	Receptor desensitization, downregulation	Dose escalation, opioid rotation
Physical dependence	Universal with chronic use (>2 weeks)	Neuroadaptation	Taper to discontinue
Hyperalgesia	5-15% of chronic use	Central sensitization, NMDA activation	Opioid rotation, dose reduction, NMDA antagonists

Opioid Switching and Equianalgesic Dosing

Drug	Oral Dose (mg) Equivalent to 10 mg IM Morphine	Conversion Factor (Oral)	Duration
Morphine	30 mg	1	4-6 hours
Oxycodone	20 mg	1.5	4-6 hours
Hydromorphone	7.5 mg	4-5	4-5 hours
Fentanyl transdermal	12 mcg/h patch ~ 45-60 mg oral morphine/day	Varies with dose	72 hours
Methadone	Varies (complex, 1:4 to 1:12 ratio depending on dose)	Unpredictable, specialist guidance required	6-24 hours
Codeine	180-200 mg	0.15	4-6 hours
Tramadol	300-400 mg	0.1-0.15	4-6 hours

Adjuvant Analgesics

Antidepressants for Pain

Drug	Mechanism	Pain Indications	Typical Dose	Onset	Key Notes
Amitriptyline (TCA)	NE + 5-HT reuptake inhibition, Na channel blockade	Neuropathic pain, fibromyalgia, migraine prophylaxis, IBS	10-75 mg qHS	1-3 weeks	Anticholinergic, sedation; start low
Nortriptyline (TCA)	NE reuptake inhibition	Neuropathic pain	25-100 mg qHS	1-3 weeks	Better tolerated than amitriptyline
Duloxetine (SNRI)	NE + 5-HT reuptake inhibition	Diabetic neuropathy, fibromyalgia, chronic MSK pain, osteoarthritis	30-60 mg daily	1-4 weeks	Also treats depression; no generic form in some countries
Venlafaxine (SNRI)	NE + 5-HT reuptake inhibition	Neuropathic pain, migraine prophylaxis	75-225 mg daily	2-6 weeks	May need higher doses for NE effect; withdrawal syndrome

Anticonvulsants for Pain

Drug	Mechanism	Pain Indications	Typical Dose	Onset	Key Notes
Gabapentin	Alpha-2-delta calcium channel modulation	Neuropathic pain (PHN, DPN), fibromyalgia	300-1200 mg TID	1-3 weeks	Renal clearance, adjust dose in renal impairment
Pregabalin	Alpha-2-delta calcium channel modulation	Neuropathic pain, fibromyalgia	75-300 mg BID	1-2 weeks	Better PK than gabapentin; dizziness/sedation
Carbamazepine	Na channel blockade	Trigeminal neuralgia (first-line)	200-400 mg BID	1-2 weeks	CYP induction, agranulocytosis risk, SIADH
Topiramate	Na channel, GABA, glutamate modulation	Migraine prophylaxis	25-200 mg BID	4-8 weeks	Weight loss, cognitive effects, nephrolithiasis

Topical Analgesics

Agent	Mechanism	Indications	Preparation	Key Notes
Lidocaine (5%)	Na channel blockade	Postherpetic neuralgia (first-line), focal neuropathic pain	Patch, cream, gel	Minimal systemic absorption (patch); apply 12h on/12h off
Capsaicin (0.025-0.075%, 8% patch)	TRPV1 agonism, substance P depletion	Neuropathic pain, osteoarthritis	Cream, patch (8% single application)	Initial burning sensation; 8% patch requires professional application
Diclofenac (1-3%)	COX inhibition	Osteoarthritis (knee/hand), soft tissue injury	Gel, patch, solution	Systemic absorption low; effective for localized OA
NSAID combinations	Various	Localized MSK pain	Various	Counterirritants (menthol, camphor) provide distraction

WHO Analgesic Ladder

Step	Pain Severity	Medications	Adjuvants
Step 1	Mild (1-3/10)	Non-opioid (acetaminophen or NSAID)	+/- Adjuvant (anticonvulsant, antidepressant, topical)
Step 2	Mild-Moderate (4-6/10)	Weak opioid (codeine, tramadol) + non-opioid	+/- Adjuvant
Step 3	Moderate-Severe (7-10/10)	Strong opioid (morphine, oxycodone, hydromorphone) + non-opioid	+/- Adjuvant

Multimodal Analgesia

Component	Examples	Benefits
Acetaminophen	Scheduled around-the-clock	Opioid-sparing, safe
NSAID	Scheduled around-the-clock	Opioid-sparing, anti-inflammatory
Regional anesthesia	Nerve blocks, epidural, neuraxial	Excellent localized pain relief, reduced systemic drugs
Local infiltration	Surgical site infiltration (e.g., liposomal bupivacaine)	Postoperative pain at surgical site
Gabapentinoids	Gabapentin, pregabalin (preoperative)	Reduced postoperative opioid consumption, improved pain scores
Ketamine	Sub-anesthetic IV doses (NMDA antagonist)	Opioid-sparing, reduced chronic postoperative pain, anti-hyperalgesia
Dexamethasone	Single low-dose IV	Peroperative anti-inflammatory, antiemetic
Lidocaine infusion	IV lidocaine	Reduces postoperative pain, speeds return of bowel function
Clonidine/dexmedetomidine	Alpha-2 agonists	Reduced opioid requirement, sedation
Norepinephrine reuptake inhibitors	Duloxetine preoperatively	Reduced opioid use postoperatively

Opioid Stewardship and Risk Mitigation

Strategy	Implementation
Screening for risk	Use Opioid Risk Tool (ORT) or SOAPP-R before initiation
Treatment agreement	Document expectations, risks, monitoring plan
Prescription drug monitoring program (PDMP)	Check state database before prescribing, every 3-6 months
Urine drug testing	Baseline and random testing for adherence, diversion
Pill counts	For higher-risk patients on chronic opioid therapy
Lowest effective dose	Avoid >50 MME/day if possible; >90 MME/day higher risk
Shortest duration	Acute: 3-7 days as needed; avoid >14 days for acute pain
Avoid concurrent sedatives	Avoid benzodiazepines/GABAergics with opioids (FDA boxed warning)
Naloxone co-prescribing	For MME >50, concurrent benzodiazepines, COPD, sleep apnea, substance use disorder
Tapering when appropriate	5-10% reduction per week or slower; offer support

Conclusion

Analgesic pharmacology encompasses diverse drug classes and mechanisms targeting different pain pathways. Multimodal analgesia combining non-opioid, opioid, and adjuvant agents optimizes pain relief while minimizing adverse effects. The WHO analgesic ladder provides a framework for stepwise treatment based on pain severity. Opioid stewardship principles are essential for safe prescribing, including risk assessment, monitoring, and appropriate discontinuation strategies.