1. Clinical Overview
Ethambutol is a bacteriostatic antimycobacterial agent, classified as a first-line oral drug for the treatment of all forms of tuberculosis (TB). It is a critical component of Directly Observed Treatment, Short-course (DOTS) and other regimens under the Revised National Tuberculosis Control Programme (RNTCP), now known as the National Tuberculosis Elimination Programme (NTEP) in India. Its primary role is to prevent the emergence of resistance to other drugs, particularly rifampicin. The 600mg strength is a standard adult dose used in the continuation phase of treatment.
| Onset | Duration | Bioavailability |
|---|---|---|
| 2-4 hours for peak plasma concentration; bacteriostatic effect begins within 24 hours. | Approximately 24 hours, supporting once-daily dosing. | 75-80% following oral administration. |
2. Mechanism of Action
Ethambutol inhibits arabinosyl transferase enzymes (specifically EmbA, EmbB, and 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)
- Extra-pulmonary Tuberculosis (including TB lymphadenitis, skeletal TB, genitourinary TB, as part of a multi-drug regimen)
4. Dosage & Administration
Adult Dosage: 15-25 mg/kg body weight once daily. For the 600mg tablet: Typically used for patients weighing approximately 40-55 kg in the continuation phase. Standard NTEP regimen: 600mg daily or thrice-weekly in the continuation phase.
Administration: Administer once daily, with or without food. To be taken at the same time each day. Tablet should be swallowed whole with a glass of water. If gastrointestinal upset occurs, can be taken with food.
5. Side Effects
Common side effects may include:
- Hyperuricemia (asymptomatic)
- Nausea
- Abdominal discomfort
- Headache
- Dizziness
- Malaise
- Joint pain (arthralgia)
- Skin rash/itching
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Aluminum Hydroxide (Antacids) | Reduced absorption of ethambutol by forming complexes. Decreased serum levels and efficacy. | Moderate |
| Aminoglycosides (e.g., Streptomycin, Amikacin) | Additive risk of ototoxicity and nephrotoxicity. | Moderate |
| Other Neurotoxic drugs (e.g., Isoniazid, Chloroquine, Disulfiram) | Increased risk of peripheral neuropathy and optic neuritis. | Moderate to High |
| Zinc supplements | May chelate with ethambutol, potentially reducing its efficacy, though clinical significance is unclear. | Minor |
7. Patient Counselling
- DO take the medicine at the same time every day, as prescribed.
- DO complete the full course of therapy (6-9 months or as advised), even if you feel better.
- DO attend ALL scheduled appointments for visual acuity testing.
- DO inform all your doctors (including eye doctors) that you are taking ethambutol.
- DO report any visual changes IMMEDIATELY.
- DON'T stop taking the medicine without consulting your doctor.
- DON'T take antacids containing aluminum within 4 hours of taking ethambutol.
- DON'T drive or operate heavy machinery if you experience blurred vision or dizziness.
8. Toxicology & Storage
Overdose: Symptoms are primarily extensions of side effects: Acute onset of visual disturbances (blurring, scotomas, color blindness), nausea, vomiting, abdominal pain, dizziness, confusion, and hallucinations. In severe cases, acute renal failure 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. Do not use after the expiry date printed on the pack.