Isoniazid
A first-line tuberculosis drug, given with others for active TB and alone to prevent disease in latent infection.
Also called INH (abbreviation) or H (letter in TB regimen codes).
Drug card
- How it works
- Blocking mycolic acid synthesis in the mycobacterial cell wall
- Used for
- Active tuberculosis, with other TB drugs
- Latent TB infection, as preventive treatment
- Known for
- Peripheral neuropathy
- Hepatitis, sometimes fatal
Study it as a flashcard, and it comes back before you forget it.
Mechanism of action
How does isoniazid work?
A prodrug that mycobacteria activate; it then blocks synthesis of mycolic acids, key lipids of the mycobacterial cell wall, and kills actively growing bacilli. It also competes with vitamin B6. The liver clears it by acetylation, which is slow in about half of people, so they reach higher levels and more toxicity.
Like other first-line tuberculosis drugs: The four first-line drugs hit Mycobacterium tuberculosis in different ways: isoniazid and ethambutol stop it building its cell wall (mycolic acids and arabinogalactan), rifampicin blocks its RNA polymerase, and pyrazinamide acts on slowly dividing bacilli at acidic pH. Resistance arises fast to any one drug alone, so they are given together: all four for 2 months, then isoniazid and rifampicin for 4.
Uses
What is isoniazid used for?
- Active tuberculosis, with other TB drugs
- Latent TB infection, as preventive treatment
Side effects
Side effects of isoniazid
- Peripheral neuropathyKnown forSerious
- Hepatitis, sometimes fatalKnown forSerious
- Severe skin reactions: SJS, TEN, DRESSSerious
- Unsteady gait and tremor (cerebellar syndrome)Serious
- SeizuresSerious
- Memory problems and psychosisSerious
- Vitamin B6 deficiency and pellagra
- Lupus-like syndrome
- Agranulocytosis and other blood disordersSerious
- High blood sugar
- Breast enlargement in men
- PancreatitisSerious
Known for marks what an exam expects you to know it for.
Watch for
Cautions and what sets it apart
- Check liver enzymes before and during treatment from age 35, and stop it if hepatitis develops.
- Give pyridoxine to people who are malnourished or prone to neuropathy, such as those with diabetes or heavy alcohol use.
- Raises phenytoin, carbamazepine and theophylline levels.
- Weak MAO inhibition: tyramine-rich foods such as cheese and red wine, or histamine-rich fish, can cause reactions.
Good to know: Never used alone for active TB, as resistance develops quickly; on its own it is a classic preventive treatment for latent infection.
Remember it
A way to remember it
RIPE
- R
- Rifampicin: orange-red urine, sweat and tears
- I
- Isoniazid: peripheral neuropathy and hepatitis
- P
- Pyrazinamide: high uric acid and gout
- E
- Ethambutol: eye damage (optic neuritis)
RIPE names the four first-line drugs, and each letter carries the side effect that drug is best known for.
A mnemonic widely used in teaching
Its class
First-line tuberculosis drugs
Other first-line tuberculosis drugs with a card:
Easily mixed up with fluoroquinolones.
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- Isoniazid oral solution: label (DailyMed)
- Isoniazid (NCI Thesaurus)
- Treatment for Latent Tuberculosis Infection (CDC)
For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.
Antibiotics: DNA, folate and tuberculosis
Isoniazid is one of 11 drugs practiced together here: their classes, how they work, what they treat and what they're known for.
Start Practicing