1. Clinical Overview
A fixed-dose combination (FDC) of Gabapentin, an anticonvulsant and analgesic, and Nortriptyline, a tricyclic antidepressant (TCA). This combination is primarily used for the management of neuropathic pain, including diabetic neuropathy and post-herpetic neuralgia, by targeting multiple pain pathways. It is a rational pharmacotherapy for pain not adequately controlled by monotherapy.
| Onset | Duration | Bioavailability |
|---|---|---|
| Gabapentin: 2-3 hours for pain relief. Nortriptyline: Analgesic effects may be seen within 1-2 weeks; antidepressant effects may take 4-6 weeks. | Gabapentin: 5-7 hours (requires TID dosing). Nortriptyline: Long half-life allows for once or twice-daily dosing. | Gabapentin: ~60%, decreases with increasing dose (saturable absorption). Nortriptyline: ~50%. |
2. Mechanism of Action
The combination provides synergistic analgesia for neuropathic pain via complementary mechanisms. Gabapentin binds to the α2δ-1 subunit of voltage-gated calcium channels in the central nervous system, reducing the release of excitatory neurotransmitters (glutamate, substance P). Nortriptyline primarily inhibits the reuptake of norepinephrine (and to a lesser extent, serotonin) at presynaptic neurons, enhancing descending inhibitory pain pathways in the brainstem and spinal cord. It also exhibits sodium channel blockade and NMDA receptor antagonism at higher doses.
3. Indications & Uses
- Management of neuropathic pain (e.g., Diabetic Peripheral Neuropathy, Post-herpetic Neuralgia)
- Chronic pain syndromes with neuropathic component
4. Dosage & Administration
Adult Dosage: Initially: One tablet (Gabapentin 100mg + Nortriptyline 10mg) at bedtime. Titrate gradually based on response and tolerability. Usual maintenance: 1 tablet in the morning and 1-2 tablets at bedtime. Maximum recommended daily dose of this FDC: Gabapentin 900mg + Nortriptyline 90mg (i.e., 9 tablets), but often limited by side effects.
Administration: Take with or without food. Swallow whole with a glass of water. The bedtime dose helps mitigate daytime sedation and improves sleep. Do not crush or chew. Avoid abrupt discontinuation; taper over at least 1 week to prevent withdrawal (nortriptyline) or seizure risk (gabapentin).
5. Side Effects
Common side effects may include:
- Sedation/Drowsiness
- Dizziness/Unsteadiness
- Dry mouth
- Constipation
- Weight gain
- Peripheral edema (gabapentin)
- Fatigue
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Monoamine Oxidase Inhibitors (Phenelzine, Tranylcypromine) | Risk of hypertensive crisis, serotonin syndrome, hyperpyrexia. Contraindicated. | High |
| Other CNS Depressants (Alcohol, Opioids, Benzodiazepines) | Additive sedation, respiratory depression, impaired motor skills. | High |
| Anticholinergics (Atropine, Oxybutynin, TCAs) | Enhanced anticholinergic effects (dry mouth, urinary retention, constipation, confusion). | Moderate |
| CYP2D6 Inhibitors (Fluoxetine, Paroxetine, Quinidine) | Increased nortriptyline plasma levels, risk of toxicity. | Moderate |
| CYP2D6 Inducers (Rifampicin, Carbamazepine) | Decreased nortriptyline plasma levels, reduced efficacy. | Moderate |
| Antihypertensives | Nortriptyline may antagonize effects of guanethidine, clonidine. Potentiates orthostatic hypotension. | Moderate |
| Anticoagulants (Warfarin) | Nortriptyline may increase INR; monitor. | Low |
7. Patient Counselling
- DO take the medication exactly as prescribed by your doctor.
- DO take the bedtime dose to help with sleep and reduce daytime sleepiness.
- DO report any new or worsening mood changes, anxiety, or suicidal thoughts immediately.
- DO inform all your doctors and dentists you are taking this medicine.
- DO NOT stop taking this medicine suddenly. It must be tapered off under medical supervision.
- DO NOT consume alcohol while on this medication.
- DO NOT drive or operate machinery until you know how it affects you.
- DO NOT take any other medicine (including OTC, herbal) without consulting your doctor.
8. Toxicology & Storage
Overdose: Gabapentin: Severe drowsiness, slurred speech, lethargy, diarrhea. Nortriptyline Dominant: Cardiac arrhythmias (tachycardia, conduction delays, ventricular fibrillation), severe hypotension, seizures, CNS depression (coma), anticholinergic crisis (hyperthermia, dilated pupils, dry skin), respiratory depression. Combined overdose is life-threatening.
Storage: Store below 30°C in a cool, dry place, protected from light and moisture. Keep out of reach of children and pets. Do not use after the expiry date printed on the pack. Dispose of unused medicine properly; do not flush.