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Isoniazid
📖 About
Always check the BNF link here.
- 💊 Most active first-line drug for treating tuberculosis (TB).
- ⚡ Potent enzyme inhibitor.
⚙️ Action
- 🦠 Bactericidal – inhibits synthesis of mycolic acids, essential for mycobacterial cell walls.
- 💉 Well absorbed, widely distributed (including CSF).
🧪 Properties
- 🔗 Structurally similar to pyridoxine (vitamin B6).
- 💧 Crosses blood–brain barrier, good CSF penetration.
- 🧬 Inactivated by hepatic acetylation (genetically variable).
💉 Indications
- 🏥 First-line treatment of tuberculosis (part of combination therapy).
🔄 Interactions
- ⏱️ Pharmacogenetics: • Rapid acetylators → ↓ drug levels, t½ ~1 hr. • Slow acetylators → ↑ drug levels, t½ ~3 hr → more toxicity.
- ⚠️ Increases toxicity of phenytoin, carbamazepine, and warfarin.
⚠️ Cautions
- 🚫 Use with caution in porphyria.
⛔ Contraindications
- 🧪 Significant liver disease.
- 🚫 Hypersensitivity to isoniazid.
💥 Side Effects
- 🌡️ Rash, fever, drug-induced lupus.
- 🧠 Neuro: peripheral neuropathy (more common in slow acetylators), memory loss, psychosis, delirium, seizures, optic neuritis (rare).
- 🩸 Haematological: agranulocytosis (rare).
- 🧪 Hepatitis: anorexia, nausea, vomiting, RUQ pain – can be fatal → stop drug immediately if suspected.
⚡ Seizures
- If due to isoniazid toxicity → requires high-dose pyridoxine (vitamin B6).
- 🚑 Always seek urgent expert help.
📊 Adult Dose
- 💊 Standard: 3–5 mg/kg/day (usually 300 mg PO OD).
- 📌 Larger doses may be required in resistant cases (specialist advice).
- 🍽️ Take on an empty stomach for best absorption.
- 💊 Pyridoxine 10–20 mg daily co-prescribed to prevent neuropathy.
- 🧪 Monitor FBC and LFTs during therapy.
| The content on this website is provided for educational and informational purposes only to support exam preparation (e.g., MLA, MRCP, USMLE) and learning. This is NOT medical advice, diagnosis, treatment, or professional guidance. It does not replace consultation with a qualified healthcare professional, official guidelines (e.g., NICE, GMC, BNF), or supervised clinical practice. Always verify information with current, authoritative sources. Makindo and its contributors accept no liability for any reliance on this content, including errors, omissions, or any resulting harm, loss, or consequences. By using this site, you agree to these terms. |
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