Isoniazid
INH
Pronunciation: eye-soe-NYE-a-zid
What it isInhA inhibitor that blocks mycobacterial mycolic-acid synthesis; active and latent tuberculosis.
Why this oneIt is strongly bactericidal against replicating bacilli and remains the backbone of latent-TB therapy.
What limits it⚑ Hepatitis risk rises with age and alcohol; give pyridoxine when neuropathy risk is present and review CYP interactions.
FDA-approved for
- tuberculosis (~1952)
FDA label
Read the full label on DailyMed ↗
| FDA dosing | Population | Start | Target | Max |
|---|---|---|---|---|
| Active tuberculosis (with other agents) | Adult | 5 mg/kg up to 300 mg QD | 15 mg/kg up to 900 mg 2-3x/week (intermittent) | |
| Active tuberculosis (with other agents) | Children | 10-15 mg/kg up to 300 mg QD | 20-40 mg/kg up to 900 mg 2-3x/week (intermittent) | |
| TB preventive therapy | Adults >30 kg | 300 mg QD | 20-30 mg/kg up to 900 mg twice weekly | |
| TB preventive therapy | Infants/children | 10 mg/kg up to 300 mg QD | 20-30 mg/kg up to 900 mg twice weekly |
Renal Undefined
Hepatic Undefined
Instructions
- Give as a single daily dose
- Concomitant pyridoxine (B6) recommended in the malnourished and those predisposed to neuropathy (alcoholics, diabetics)
- Intermittent (2-3x/week) regimens should be given by directly observed therapy
Available strengths
- tab 100, 300 mg
- sol 50 mg/5 mL
- inj 100 mg/mL
Contraindications
- Previous isoniazid-associated hepatic injury
- Prior severe reaction to isoniazid (drug fever, chills, arthritis)
- Acute liver disease of any etiology
Cautions
- Hepatotoxicity, including fatal hepatitis (see boxed warning)
- Dose-related peripheral neuropathy, esp. malnourished, alcoholics, diabetics
- Severe cutaneous reactions (SJS/TEN, DRESS, AGEP)
- CNS effects: seizures, optic neuritis, toxic encephalopathy/psychosis
- Hematologic effects; hyperglycemia; metabolic acidosis
Adverse reactions
- Peripheral neuropathy
- Elevated transaminases/hepatitis
- Nausea, vomiting, epigastric distress
- Hematologic: agranulocytosis, anemias, thrombocytopenia, eosinophilia
- Hypersensitivity rash and fever
- Pyridoxine deficiency/pellagra
Pregnancy Category C; used for active TB when benefit justifies risk; crosses placenta — observe neonate.
Lactation Low levels in breast milk; not considered to threaten the neonate (per pregnancy section).
Cost
| Strength | #/mo | generic $/mo | CostPlus | |
|---|---|---|---|---|
| 100 mg | tab | 30 qd | $51.97 | offered |
| 300 mg | tab | 30 qd | $102.06 | out of stock |
Generic entry1982
⚠ BOXED WARNING
- Severe and sometimes fatal hepatitis
Classes and tags
Clinical profile
Legacy pregnancy categoryC
Defining liabilityHepatotoxicity and neurotoxicity; pyridoxine is needed in at-risk patients.
Serotonin and MAOI
Provisional washout figures
Elevating mechanismWeak/uncertain MAO-A inhibitor
Non-elevating actionNone recorded
Exposure dependenceUncertain/exposure dependent
Washout AFTER an MAOIU days
Evidence confidenceLow
InterpretationWeak/uncertain nonselective MAOI activity; evidence is limited and confounded.