Isoniazid

Aka: Isoniazid, INH

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II. Indications

  1. Tuberculosis Prophylaxis
  2. Multi-drug Tuberculosis Treatment regimen

III. Background

  1. Structurally similar to Pyridoxine
  2. Interferes with formation of Mycobacteria cell walls

IV. Contraindications

  1. G6PD Deficiency
  2. Severe hepatic insufficiency (reduce INH dose)
  3. Active hepatitis
  4. End-stage liver disease

V. Dosing

  1. See Tuberculosis for specific dosing protocols and durations of treatment
  2. Daily dosing
    1. Adult: 300 mg PO or IM
    2. Child: 5 mg/kg (up to 300 mg daily)
  3. Intermittent dosing
    1. Adult: 900 mg orally twice weekly (15 mg/kg twice weekly)

VI. Adverse Effects: Hepatic

  1. Hepatotoxicity
    1. Subclinical Hepatitis: 10%
    2. Clinical Hepatitis: 1%
    3. Risk increases with age and Alcohol intake
  2. Progressive liver damage
    1. Under age 20 years: rare
    2. Age 30 to 50 years: 1.5%
    3. Age over 50 years: 2.5%
  3. Common presenting symptoms of drug-induced hepatotoxicity
    1. Nausea or Vomiting
    2. Abdominal Pain (>50% of patients)
    3. Fever (10%)
    4. Rash (5%)

VII. Adverse Effects: Neurologic

  1. Peripheral Neuropathy
    1. See prevention below
  2. Insomnia
  3. Restlessness
  4. Muscle Twitching
  5. Urinary Retention
  6. Seizures
  7. Psychosis
  8. Monoamine toxicity (MAO Inhibitor-like reaction)
    1. Presents with Flushing
    2. Avoid concurrent aged cheese and wine

VIII. Adverse Effects: Miscellaneous

  1. Lupus-Like Syndrome
  2. Hypersensitivity Reactions
    1. Hemolytic Anemia
    2. Stevens-Johnson Syndrome

IX. Prevention: Peripheral Neuropathy

  1. Indications for concurrent Vitamin B6 supplementation (Neuropathy risks)
    1. Pregnancy
    2. Breast Feeding infants
    3. Diabetes Mellitus
    4. Alcoholism
    5. Comorbid other Neuropathy
    6. Uremia
    7. Malnutrition
    8. HIV Infection
    9. Epilepsy
  2. Vitamin B6 (Pyridoxine)
    1. Pyridoxine 10 mg/day per 100 mg of INH
    2. Typical dose is 25-50 mg orally daily

X. Drug Interactions

  1. Phenytoin (Dilantin)
    1. INH Increases blood level and toxicity
  2. Disulfiram (Antabuse)

XI. Monitoring: Liver Function Tests

  1. Significant liver function abnormalities exceed 3-5 times normal with Isoniazid
  2. Baseline Liver Function Tests
    1. Controversial (some experts do not recommend baseline LFTs)
    2. Others recommend baseline testing and then monthly if abnormal baseline LFTs or if otherwise indicated (as below)
  3. Obtain monthly Liver Function Tests if:
    1. Abnormal baseline Liver Function Test
    2. History suggestive of Possible liver disorder
    3. Human Immunodeficiency Virus (HIV)
    4. Pregnancy
    5. Postpartum (within 3 months)
    6. Chronic Liver Disease
      1. Viral Hepatitis (Hepatitis B or Hepatitis C)
      2. Alcoholic Hepatitis
      3. Cirrhosis

XII. References

  1. (2000) MMWR Recomm Rep 49(RR-6): 1-51 [PubMed]

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Cost: Medications

isoniazid (on 12/21/2022 at Medicaid.Gov Survey of pharmacy drug pricing)
ISONIAZID 300 MG TABLET Generic $0.20 each
FPNotebook does not benefit financially from showing this medication data or their pharmacy links. This information is provided only to help medical providers and their patients see relative costs. Insurance plans negotiate lower medication prices with suppliers. Prices shown here are out of pocket, non-negotiated rates. See Needy Meds for financial assistance information. GoodRx is another tool for finding lower drug prices.

Related Topics in Pharmacology

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