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, particularly diabetic peripheral neuropathy and postherpetic neuralgia, by targeting multiple pain pathways. It is a rational pharmacotherapy for pain not adequately controlled by monotherapy.
| Onset | Duration | Bioavailability |
|---|---|---|
| Gabapentin: Pain relief may begin within 1-2 weeks. Nortriptyline: Analgesic effects may be seen in 1-2 weeks; antidepressant effects take 4-6 weeks. | Gabapentin: 6-8 hours (requires TID dosing). Nortriptyline: 24 hours (allows for once-daily dosing). | Gabapentin: ~60%, decreases with increasing dose due to saturable L-amino acid transporter absorption. Nortriptyline: ~50%. |
2. Mechanism of Action
The combination provides synergistic analgesia for neuropathic pain via distinct and 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 nerve terminals, enhancing descending inhibitory pain pathways in the spinal cord. It also exhibits sodium channel blockade and NMDA receptor antagonism.
3. Indications & Uses
- Management of Neuropathic Pain (Diabetic Peripheral Neuropathy)
- Postherpetic Neuralgia (PHN)
- Chronic Pain Syndromes with neuropathic component
4. Dosage & Administration
Adult Dosage: Usually 1 tablet (Gabapentin 300mg + Nortriptyline 10mg) once daily at bedtime. May be titrated based on response and tolerability. Maximum: Gabapentin 1800-3600mg/day in divided doses; Nortriptyline 75-100mg/day. Titration should be slow, over weeks.
Administration: Administer orally with or without food. Taking at bedtime is recommended to minimize daytime sedation and dizziness. Swallow whole with a glass of water. Do not crush or chew. If a dose is missed, take it as soon as remembered unless it's close to the next dose. Do not double the dose.
5. Side Effects
Common side effects may include:
- Sedation/Somnolence
- Dizziness/Lightheadedness
- Dry mouth
- Constipation
- Weight gain
- Fatigue
- Peripheral edema (Gabapentin)
- Blurred vision
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Monoamine Oxidase Inhibitors (MAOIs) - Phenelzine, Tranylcypromine | Risk of serotonin syndrome, hyperpyrexia, seizures, death. | Contraindicated |
| Other CNS Depressants (Alcohol, Opioids, Benzodiazepines, Barbiturates) | Additive CNS depression, respiratory depression, sedation, impaired motor skills. | Major |
| Anticholinergic drugs (Atropine, Antihistamines, Antipsychotics) | Increased anticholinergic side effects (dry mouth, constipation, urinary retention, 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 (Clonidine) | Nortriptyline may antagonize antihypertensive effect. | Moderate |
| Sympathomimetics (Adrenaline, Noradrenaline) | Enhanced pressor response, risk of hypertension, arrhythmias. | Moderate |
| Warfarin | Nortriptyline may alter anticoagulant effect; monitor INR. | Moderate |
7. Patient Counselling
- DO take the medicine exactly as prescribed, usually at bedtime.
- DO report any thoughts of self-harm or worsening depression immediately.
- DO inform all your doctors and dentists you are taking this medicine.
- DO rise slowly from sitting/lying position to avoid dizziness.
- DO maintain good oral hygiene to manage dry mouth.
- DON'T stop taking the medicine abruptly; taper under doctor's guidance.
- DON'T consume alcohol or sleep-inducing OTC medicines.
- DON'T drive or operate machinery until you know how it affects you.
- DON'T take any new medicine (including herbal) without consulting your doctor.
8. Toxicology & Storage
Overdose: Gabapentin: Severe drowsiness, dizziness, slurred speech, lethargy, diarrhea. Nortriptyline: Predominates in overdose: Cardiac arrhythmias (tachycardia, conduction delays, ventricular fibrillation), severe hypotension, seizures, respiratory depression, coma, anticholinergic crisis (hyperthermia, flushed skin, dilated pupils, ileus), confusion, hallucinations. Combined overdose is a medical emergency.
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.