Pharmacology · Antituberculosis drugs
Ethionamide !
A second-line TB drug related to isoniazid that blocks the same enzyme, InhA. It is poorly tolerated: stomach upset, liver injury and hypothyroidism.
Updated 2026-11-01
Also called: Eto
The figure shows the ethionamide–NAD adduct (its active form after EthA activation) bound in InhA of M. tuberculosis.
Mechanism of action
Ethionamide is a prodrug. The mycobacterial monooxygenase EthA activates it, and the active form inhibits InhA, the same enzyme of mycolic acid synthesis that isoniazid blocks. Strains that overproduce InhA can show low-level resistance to both drugs.
- EthA activates ethionamide inside the bacillus
- The active form blocks InhA
- Mycolic acid synthesis stops
Pharmacokinetics
- Absorption
- Nearly complete by mouth
Indications
- Multidrug- or rifampicin-resistant tuberculosis, only when bedaquiline, linezolid, clofazimine or delamanid cannot be used Tuberculosis →
Adverse effects
Common
- Nausea, vomiting, loss of appetite, metallic taste
- Drowsiness, low mood, dizziness, postural hypotension
- Gynaecomastia, hair loss, acne, impotence, menstrual changes
Contraindications
- Hypersensitivity to ethionamide
Key interactions
- Cycloserine
- Adds to nervous system toxicity
What is monitored
- Liver tests at regular intervals
- Thyroid function
- Mood and neurological symptoms
- Food intake and weight
Administration and safety
- By mouth; taking it with food or at bedtime eases stomach upset
- Pyridoxine is given with it to prevent neurological effects
Nursing care
- Ask about nausea and appetite; intolerance is the main reason it is stopped
- Teach the signs of hepatitis
- Watch for tiredness, cold intolerance or weight gain (hypothyroidism)
- Check that pyridoxine is given
Test yourself
Think of the answer, then tap the question to see it.
Which enzyme do ethionamide and isoniazid share as target?
InhA, the enoyl-ACP reductase of mycolic acid synthesis.
Which endocrine effect of ethionamide is checked with blood tests?
Hypothyroidism, which is reversible.
Where does ethionamide fit in current WHO guidance?
Only in longer MDR/RR-TB regimens when bedaquiline, linezolid, clofazimine or delamanid cannot be used.
Related drugs
Related conditions
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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: WHO consolidated guidelines on tuberculosis: module 4: treatment and care (World Health Organization (WHO), 2025); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 65 (McGraw Hill, 2023).
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Representative model, for educational purposes only.