1. Clinical Overview
Ethambutol is a bacteriostatic antimycobacterial agent, classified as a first-line drug for the treatment of all forms of tuberculosis (TB). It is a synthetic, water-soluble, heat-stable compound, administered orally. Its primary clinical value lies in its ability to delay the emergence of resistance to other first-line drugs, particularly isoniazid and rifampicin, when used in combination therapy. The 1000mg strength is a standard adult dose used in the intensive phase of treatment under the Revised National Tuberculosis Control Programme (RNTCP), now known as the National Tuberculosis Elimination Programme (NTEP) in India.
| Onset | Duration | Bioavailability |
|---|---|---|
| 2-4 hours (peak plasma concentration) | 24 hours (allows once-daily dosing) | 70-80% |
2. Mechanism of Action
Ethambutol inhibits arabinosyl transferase enzymes (EmbA, EmbB, EmbC) which are crucial for the polymerization of cell wall arabinogalactan, a major component of the mycobacterial cell wall. This inhibition disrupts the synthesis of the mycobacterial cell wall, leading to impaired cell wall integrity and bacteriostatic activity.
3. Indications & Uses
- Pulmonary Tuberculosis (as part of a multi-drug regimen)
- Extrapulmonary Tuberculosis (including TB meningitis, skeletal TB, genitourinary TB)
4. Dosage & Administration
Adult Dosage: 15-25 mg/kg body weight once daily. Standard NTEP/WHO recommended dose for intensive phase: 15 mg/kg/day (e.g., 1000mg for patients weighing 55-70 kg). For retreatment cases or where drug resistance is suspected: 25 mg/kg/day for first 2 months.
Administration: Administer once daily, with or without food. Tablet should be swallowed whole with water. To be taken at the same time each day, preferably in the morning. Always as part of a combination regimen (e.g., with Isoniazid, Rifampicin, Pyrazinamide).
5. Side Effects
Common side effects may include:
- Hyperuricemia (asymptomatic)
- Gastrointestinal upset (nausea, abdominal pain)
- Headache
- Dizziness
- Malaise
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Aluminum hydroxide antacids | Decreased absorption of Ethambutol by up to 20-30%. Administer at least 4 hours apart. | Moderate |
| Other neurotoxic drugs (e.g., Chloroquine, Isoniazid, Linezolid) | Increased risk of peripheral neuropathy and optic neuritis. Enhanced monitoring required. | Major |
| Allopurinol/Probenecid | May alter uric acid levels. Monitoring of serum uric acid recommended. | Moderate |
| Rifampicin | No significant PK interaction. They are used together synergistically in TB therapy. | None |
7. Patient Counselling
- DO take the medicine exactly as prescribed, at the same time daily.
- DO NOT stop taking the medicine even if you feel better, unless instructed by your doctor.
- DO attend ALL follow-up appointments for sputum tests, vision checks, and clinical evaluation.
- DO inform all your doctors (including eye doctors) that you are on Ethambutol.
- DO NOT take aluminum-containing antacids within 4 hours of Ethambutol dose.
8. Toxicology & Storage
Overdose: Symptoms are primarily related to its major toxicities: Acute onset of visual disturbances (blurred vision, central scotomas, color blindness), nausea, vomiting, dizziness, confusion, and abdominal pain. In severe overdose, hallucinations and metabolic acidosis may occur.
Storage: Store below 30°C, in a cool, dry place, protected from light and moisture. Keep the container tightly closed. Keep out of reach of children.