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Explain ADR

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isoniazid AND adverse effects

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CDC isoniazid adverse effects peripheral neuropathy hepatitis pyridoxine

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ADR means Adverse Drug Reaction: a harmful, unintended effect occurring when a medicine is used at normal therapeutic doses.
In your image, ADRs of isoniazid (INH) are:
  1. Hepatitis / hepatotoxicity
    • The most serious ADR.
    • May progress to fatal liver failure if INH is continued despite hepatitis.
    • Risk is higher in people aged over 35 years, those taking rifampicin too, and people who drink alcohol daily.
    • Warning symptoms: loss of appetite, nausea/vomiting, unusual tiredness, dark urine, jaundice, or right upper abdominal pain. Seek medical care promptly if these occur.
  2. Peripheral neuropathy
    • Presents as tingling, numbness, burning, or pins-and-needles sensations in the hands and feet.
    • INH causes a relative pyridoxine (vitamin B6) deficiency.
    • Prevented with pyridoxine supplementation, particularly in people at higher risk such as those with diabetes, alcohol use disorder, malnutrition, HIV, kidney failure, pregnancy, or pre-existing neuropathy.
  3. Central nervous system effects
    • May cause irritability, insomnia, confusion, or rarely convulsions/seizures, especially in seizure-prone patients.
  4. Hypersensitivity reactions
    • Rash and fever can occur.
  5. Drug interactions
    • INH inhibits metabolism of phenytoin and carbamazepine, so their blood concentrations may rise.
    • This can increase toxicity, for example:
      • Nystagmus: involuntary rhythmic eye movements
      • Ataxia: unsteady gait and poor coordination
Easy mnemonic: INH
  • I: Injury to liver
  • N: Neuropathy from B6 deficiency
  • H: Hypersensitivity, seizures, and higher phenytoin/carbamazepine toxicity
This matches Lippincott Illustrated Reviews: Pharmacology, pp. 1057-1058.
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