A multimodal approach aims to achieve additive or synergistic analgesia while minimizing the dose-dependent side effects of any single agent—particularly opioids. This article reviews the primary classes of non-opioid medications used in chronic and neuropathic pain management.

1. Gabapentinoids (Calcium Channel Blockers)

Despite their name, gabapentin and pregabalin do not act directly on GABA receptors.

• Mechanism of Action: They bind to the alpha_2 delta subunit of voltage-gated calcium channels in the CNS. By binding here, they inhibit the influx of calcium into the presynaptic terminal, which subsequently reduces the release of excitatory neurotransmitters like glutamate, substance P, and norepinephrine.
• Kinetics: * Gabapentin: Exhibits zero-order (saturable) absorption. As the dose increases, the percentage of the drug absorbed decreases.
  • Pregabalin: Exhibits first-order (linear) absorption, making its bioavailability more predictable.
• Side Effects: Sedation, dizziness, peripheral edema, and weight gain.
• Board Pearl: Both require renal dose adjustment. They are first-line for Diabetic Peripheral Neuropathy (DPN) and Post-Herpetic Neuralgia (PHN).

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2. Antidepressants: TCAs and SNRIs

These medications leverage the Descending Inhibitory Pathways. By increasing the concentration of neurotransmitters in the synaptic cleft of the spinal cord, they enhance the body’s natural pain-dampening system.

Tricyclic Antidepressants (TCAs)

• Examples: Amitriptyline, Nortriptyline (a metabolite of amitriptyline).
• Mechanism: Inhibit the reuptake of both Serotonin (5-HT) and Norepinephrine (NE). They also possess sodium channel-blocking properties.
• Side Effects: Significant anticholinergic effects (dry mouth, urinary retention, constipation) and antihistaminergic effects (sedation).
• Board Pearl: Use with extreme caution in the elderly (Beers Criteria) and patients with cardiac conduction issues (can cause QTc prolongation). Nortriptyline is generally better tolerated than Amitriptyline.

Serotonin-Norepinephrine Reuptake Inhibitors (SNRIs)

• Examples: Duloxetine, Milnacipran, Venlafaxine.
• Mechanism: Potent inhibition of 5-HT and NE reuptake.
• Indications: Duloxetine is FDA-approved for DPN, Fibromyalgia, and Chronic Musculoskeletal Pain (like OA of the knee or chronic low back pain).
• Board Pearl: SNRIs are preferred over TCAs in patients with cardiac risks or those who cannot tolerate anticholinergic side effects.

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3. NMDA Receptor Antagonists

NMDA receptor is the “gatekeeper” of central sensitization.

• Examples: Ketamine, Memantine, Dextromethorphan.
• Mechanism: Non-competitive antagonism of the N-methyl-D-aspartate (NMDA) receptor, preventing the calcium influx associated with “wind-up.”
• Clinical Use: Ketamine infusions are used for refractory neuropathic pain and CRPS. It is also highly effective in reducing perioperative opioid consumption.
• Side Effects: Dissociative symptoms, hallucinations, hypertension, and tachycardia.

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4. Sodium Channel Blockers (Membrane Stabilizers)

• Systemic: Mexiletine (an oral analog of Lidocaine).
• Topical: Lidocaine 5% patch.
• Mechanism: They bind to voltage-gated sodium channels, preventing the propagation of action potentials in hyperexcitable or damaged nerves.
• Board Pearl: The Lidocaine 5% patch is specifically FDA-approved for PHN. It acts locally, and systemic absorption is minimal, making it safe for patients with multiple comorbidities.

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5. Non-Steroidal Anti-Inflammatory Drugs (NSAIDs)

• Mechanism: Inhibition of the Cyclooxygenase (COX) enzymes, preventing the conversion of arachidonic acid into prostaglandins.
  • COX-1: Constitutively expressed; maintains gastric mucosa and platelet function.
  • COX-2: Induced during inflammation.
• Selective COX-2 Inhibitors (Celecoxib): Designed to reduce GI side effects but carry a theoretical increased risk of cardiovascular events due to the inhibition of prostacyclin without inhibiting thromboxane.
• Board Pearl: Chronic NSAID use is limited by risks of GI bleeding, renal impairment (afferent arteriole constriction), and interference with aspirin’s antiplatelet effect.

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6. Skeletal Muscle Relaxants

These are a heterogeneous group of drugs used for muscle spasms or spasticity.

• Antispasticity Agents: Baclofen (GABA-B agonist), Dantrolene (acts directly on the sarcoplasmic reticulum).
• Antispasmodic Agents: Cyclobenzaprine (structurally similar to TCAs), Tizanidine (alpha_2-adrenergic agonist similar to clonidine), Carisoprodol (Schedule IV; metabolized to meprobamate, high abuse potential).