Myambutol (ethambutol hydrochloride) is a first-line bacteriostatic antimycobacterial agent specifically indicated for the treatment of pulmonary tuberculosis. It is always used in conjunction with other antitubercular drugs such as isoniazid, rifampin, and pyrazinamide as part of a multi-drug regimen to prevent the development of drug-resistant strains. Its primary mechanism of action involves the inhibition of arabinosyl transferase, an enzyme critical for the synthesis of the mycobacterial cell wall, thereby halting bacterial replication. This targeted action makes it a cornerstone in both initial and continuation phases of standard TB therapy, particularly effective against actively dividing microorganisms.

Features

• Active pharmaceutical ingredient: Ethambutol Hydrochloride
• Available in scored, film-coated tablet formulations for accurate dosing (100 mg, 400 mg)
• Bacteriostatic action against Mycobacterium tuberculosis
• High oral bioavailability, not significantly affected by food
• Primarily renal excretion, with a half-life of approximately 3-4 hours (prolonged in renal impairment)
• Distributed widely in body tissues and fluids, including cerebrospinal fluid in inflamed meninges

Benefits

• Prevents Resistance: Essential component of combination therapy, drastically reducing the risk of emergent drug-resistant tuberculosis.
• Targeted Action: Specifically inhibits cell wall synthesis in mycobacteria, sparing host cells and reducing broad-spectrum side effects.
• Dosage Flexibility: Scored tablets and weight-based dosing protocols allow for precise individualization of therapy.
• Proven Efficacy: Decades of clinical use and inclusion in WHO treatment guidelines confirm its role in successful TB eradication.
• Oral Administration: Facilitates outpatient treatment and improves adherence compared to injectable regimens.

Common use

Myambutol is exclusively used for the treatment of all forms of pulmonary tuberculosis. It is never used as monotherapy due to the rapid development of resistance. Its use is standard in the initial two-month phase of treatment alongside isoniazid, rifampin, and pyrazinamide. It may be continued into the continuation phase (typically four additional months) in cases where drug susceptibility testing confirms its ongoing necessity or in specific clinical scenarios as determined by an infectious disease or pulmonary specialist. Its use is also considered in the treatment of some atypical mycobacterial infections, though this is off-label and requires expert consultation.

Dosage and direction

Dosage is strictly based on patient body weight and must be calculated at the initiation of therapy and re-evaluated with significant weight change.

• Adults and Children >12 years: 15 to 25 mg/kg (ideal body weight) administered orally once daily. The most common daily dose is 15 mg/kg. Higher doses (25 mg/kg) may be used initially in patients with suspected or confirmed resistance, but only under direct specialist supervision and typically for a limited duration (e.g., first 2 months).
• Children <12 years: 15 to 20 mg/kg/day (max 1 gram daily). Use in young children requires extreme caution due to the difficulty in monitoring for ocular toxicity.
• Dosing in Renal Impairment: Dose adjustment is mandatory. A common guideline is to administer 15-25 mg/kg dose three times per week instead of daily in patients with creatinine clearance less than 30 mL/min. Dosing must be individualized based on glomerular filtration rate (GFR).

The tablet can be taken with or without food to minimize gastrointestinal upset. Consistent daily timing is recommended to maintain stable drug levels.

Precautions

• Ophthalmic Monitoring: Baseline ophthalmological examination (including visual acuity, color discrimination, and visual fields) is required before initiation of therapy. Repeat examinations should be performed monthly during therapy. Patients must be instructed to report any visual changes (blurriness, red-green color blindness, constricted visual fields) immediately.
• Renal Function: Serum creatinine and calculation of creatinine clearance must be assessed prior to and periodically during treatment. Dose reduction is necessary in renal impairment.
• Hepatic Function: Although primarily renally excreted, hepatic function should be monitored as part of overall TB treatment management.
• Gout: May cause elevated serum uric levels, which can precipitate acute gouty arthritis.
• Pregnancy (Category C): Use only if the potential benefit justifies the potential risk to the fetus. Ethambutol crosses the placental barrier.
• Lactation: Ethambutol is excreted in human milk. The benefits of breastfeeding during therapy should be weighed against the potential risk of drug exposure to the infant.

Contraindications

Myambutol is contraindicated in patients with:

• Known hypersensitivity to ethambutol or any component of the formulation.
• Optic neuritis (unless clinical necessity outweighs extreme risk).
• Significant renal impairment where dose monitoring and adjustment cannot be reliably performed.
• In children too young to be reliably monitored for visual acuity and symptoms (generally under 5 years, though use is avoided in most young children).

Possible side effect

The most significant and characteristic adverse reaction is ocular toxicity.

• Common (>1%): Hyperuricemia, nausea, abdominal pain, headache, dizziness, rash, pruritus, joint pain (arthralgia).
• Serious:
  • Ocular: Retrobulbar neuritis, optic neuritis, manifested as decreased visual acuity, scotoma, color blindness (especially red-green discrimination), and constricted visual fields. This is dose- and duration-dependent and may be irreversible.
  • Hepatic: Hepatitis (often part of a drug-induced liver injury pattern from combination therapy).
  • Hypersensitivity: Dermatitis, pruritus, anaphylactoid reactions.
  • Peripheral neuropathy: Numbness and tingling in extremities.
  • Metabolic: Elevated uric acid, precipitating gout.

Drug interaction

• Aluminum Hydroxide: Antacids containing aluminum can significantly decrease the absorption of ethambutol, reducing its bioavailability. Dosing should be separated by at least 2-4 hours.
• Other Neurotoxic Drugs: Concomitant use with other drugs known to cause ocular or neurological toxicity (e.g., chloroquine, hydroxychloroquine, disulfiram, isoniazid) may potentiate adverse effects. Enhanced monitoring is required.
• Nephrotoxic Drugs: Drugs that impair renal function (e.g., aminoglycosides, vancomycin, NSAIDs) may decrease the clearance of ethambutol, increasing the risk of toxicity.

Missed dose

If a dose is missed, it should be taken as soon as it is remembered on the same day. If it is not remembered until the next day, the patient should skip the missed dose and resume the normal dosing schedule. The patient should never take a double dose to make up for a missed one. Maintaining a consistent schedule is critical for treatment efficacy.

Overdose

Symptoms of overdose are primarily an extension of the known adverse effects, most notably acute optic neuritis with rapid onset of visual disturbances, nausea, vomiting, abdominal pain, and dizziness. There is no specific antidote for ethambutol overdose. Management consists of immediate gastric lavage or administration of activated charcoal (if presentation is early), followed by supportive and symptomatic care. Hemodialysis can remove a significant portion of the drug from the circulation and should be considered in cases of significant overdose, especially in patients with underlying renal dysfunction.

Storage

Store Myambutol tablets at controlled room temperature, 20°C to 25°C (68°F to 77°F), in a tight, light-resistant container. Keep out of reach of children and pets. Do not use after the expiration date printed on the packaging. Do not flush medications down the toilet or pour them down a drain unless instructed to do so.

Disclaimer

This information is for educational and professional medical reference purposes only and is not a substitute for the professional judgment of a healthcare provider in diagnosing and treating patients. The prescribing physician must rely upon their own clinical experience and knowledge when making individual patient treatment decisions. The information provided here may not cover all possible uses, directions, precautions, drug interactions, or adverse effects.

Reviews

• “A critical and non-negotiable component of the RIPE (Rifampin, Isoniazid, Pyrazinamide, Ethambutol) regimen for drug-susceptible TB. Its role in preventing resistance is invaluable. The necessity for diligent visual monitoring cannot be overstated.” – Infectious Disease Specialist, 15 years experience.
• “While effective, the specter of irreversible ocular toxicity demands immense respect. Our clinic has a strict protocol for baseline and monthly visual exams. It forces a crucial dialogue about adherence and symptom reporting with every patient.” – Pulmonary Physician.
• “Dosing is straightforward with weight-based calculations, but renal function is the key to safety. It’s a drug that works well when you follow the rules meticulously.” – Clinical Pharmacist, TB Program.
• “The benefit of an oral agent with a specific mechanism of action in such a devastating disease is clear. It remains a first-line option because its benefits, when managed correctly, far outweigh its risks.” – Global Health Practitioner.