1. Clinical Overview
A fixed-dose combination of a tricyclic antidepressant (TCA), Dosulepin (Dothiepin), and an active form of Vitamin B12, Methylcobalamin. Dosulepin primarily treats depression and neuropathic pain by modulating monoamine neurotransmitters, while Methylcobalamin supports nerve health and may aid in the management of neuropathic symptoms and B12 deficiency. This combination is often used in India for depressive disorders with associated somatic symptoms, fatigue, or neuropathic pain.
| Onset | Duration | Bioavailability |
|---|---|---|
| Antidepressant effect: 2-4 weeks. Sedative effect: Within 1-2 hours. Methylcobalamin levels begin to rise within hours, but neurological benefits may take weeks to months. | Dosulepin: 24-48 hours (due to active metabolite). Methylcobalamin: Effects are sustained with regular dosing due to hepatic storage. | Dosulepin: ~30-60% (subject to significant first-pass metabolism). Methylcobalamin: Orally ~1-3% via intrinsic factor-mediated absorption; this formulation is designed for improved absorption. |
2. Mechanism of Action
Dosulepin is a potent inhibitor of presynaptic reuptake of serotonin (5-HT) and norepinephrine (NE), increasing their synaptic concentrations. It also has strong antagonistic actions at histamine H1, muscarinic acetylcholine, and alpha-1 adrenergic receptors, contributing to its side effect profile. Methylcobalamin acts as a cofactor for methionine synthase (converting homocysteine to methionine) and L-methylmalonyl-CoA mutase. It is essential for myelin synthesis, neuronal repair, and normal erythropoiesis.
3. Indications & Uses
- Major Depressive Disorder
- Neuropathic Pain (e.g., diabetic neuropathy, post-herpetic neuralgia)
4. Dosage & Administration
Adult Dosage: Initially: 75mg Dosulepin (usually as 25mg TDS or 75mg HS). Maintenance: 75-150mg/day in divided doses or as a single nocturnal dose. Methylcobalamin 1500mcg is fixed. Max daily dose of Dosulepin: 225mg.
Administration: Take after food to reduce GI upset. The entire daily dose can be taken at bedtime to capitalize on sedative effect and improve compliance. Do not crush or chew sustained-release formulations if available. Swallow whole with water.
5. Side Effects
Common side effects may include:
- Sedation/Drowsiness
- Dry mouth
- Constipation
- Blurred vision
- Weight gain
- Postural hypotension
- Increased sweating
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| MAOIs (Phenelzine, Tranylcypromine) | Serotonin syndrome, hyperpyrexia, seizures, death | Contraindicated |
| Other CNS Depressants (Alcohol, Benzodiazepines, Opioids) | Potentiated sedation, respiratory depression | Major |
| Antiarrhythmics (Quinidine, Amiodarone), Antipsychotics | Additive QT prolongation, risk of fatal arrhythmias | Major |
| Anticholinergics (Atropine, Trihexyphenidyl) | Enhanced anticholinergic toxicity (ileus, hyperthermia) | Major |
| SSRIs/SNRIs (Fluoxetine, Paroxetine) | Increased TCA levels (CYP2D6 inhibition), serotonin syndrome risk | Major |
| Clonidine | Antagonizes antihypertensive effect | Moderate |
| Warfarin | May increase anticoagulant effect | Moderate |
| Metformin, Proton Pump Inhibitors (long-term) | May reduce Methylcobalamin absorption | Moderate |
7. Patient Counselling
- DO take it exactly as prescribed, usually at bedtime.
- DO inform all doctors and dentists you are taking this medicine.
- DO continue taking even if you feel better; do not stop abruptly.
- DONT consume alcohol.
- DONT take any other medicines (including OTC) without consulting your doctor.
- DONT drive or operate heavy machinery until you know how it affects you.
- DO maintain good oral hygiene to manage dry mouth.
8. Toxicology & Storage
Overdose: Dosulepin overdose is LIFE-THREATENING. Symptoms: CNS (drowsiness, coma, seizures), Cardiac (QT prolongation, ventricular arrhythmias, heart block, hypotension), Anticholinergic (dry skin, hyperthermia, ileus, urinary retention), Respiratory depression. Methylcobalamin: No known toxicity.
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.