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Isoniazid indications, dose, side effects & concise monograph


Adult: 5 mg/kg daily. Max: 300 mg daily. For intermittent treatment: 10 mg/kg 3 times a wk or 15 mg/kg twice wkly. Similar doses may also be given via IM admin. For latent tuberculosis: 300 mg daily for 6 mth; alternatively, 5 mg/kg daily or 15 mg/kg twice wkly for 9 mth.
Child: 10-15 mg/kg/day in 1-2 divided doses (max: 300 mg/day). Alternatively, intermittent therapy can be given at 20-40 mg/kg (max: 900 mg) 2-3 times wkly. For latent tuberculosis: 10-20 mg/kg/day or 20-40 mg/kg twice wkly for 9 mth. Max: 300 mg/dose for daily regimen and 900 mg/dose for intermittent regimens.
Renal impairment: Reduce dose in severe renal impairment.
Nontuberculous mycobacterial infections
Adult: 5 mg/kg/day for at least 12 mth of culture-negative sputum; usually used with ethambutol and rifampin. Max: 300 mg/day. Similar doses may also be given via IM admin.

Special Populations: Reduce dose in patients with hepatic impairment and moderate to severe renal impairment.
Acute liver disease or history of hepatic damage during INH therapy; hypersensitivity.
Warnings / Precautions
Renal or hepatic impairment; convulsive disorders; history of psychosis; patients at risk of neuropathy or pyridoxine deficiency eg, diabetic, alcoholic, malnourished, uraemic, infected with HIV. Careful monitoring of hepatic function is necessary for black and hispanic women. Check hepatic function before and during treatment. Pregnancy and lactation.
Adverse Reactions
Peripheral neuritis, optic neuritis; psychotic reactions, convulsions, nausea, vomiting, fatigue, epigastric distress, visual disturbances, fever, rash, pyridoxine deficiency.
Potentially Fatal: Hepatotoxicity.
Overdose Reactions
Symptoms: Slurred speech, metabolic acidosis, hallucinations, hyperglycaemia, respiratory distress or tachypnoea, convulsions, coma.
Drug Interactions
May increase toxicity of carbamazepine, ethosuximide, phenytoin, diazepam and triazolam, chlorzoxazone, theophylline, clofazimine, cycloserine and warfarin. May increase metabolism of enflurane resulting in nephrotoxic fluoride levels. Reduced efficacy and increased risk of peripheral neuropathies and hepatotoxicity with alcohol. Reduced absoprtion with aluminium-containing antacids; give at least 1 hr before the antacid. Decreased serum levels with ketoconazole, zalcitabine. Increased risk of peripheral neuropathy with stavudine and zalcitabine.
Potentially Fatal: Increased risk of hepatotoxicity with rifampicin and other hepatotoxic drugs.
See Below for More isoniazid Drug Interactions
Lab Interactions
False-positive urinary glucose with cupric sulfate solution.
Food Interactions
Decreased serum levels if taken with food. Avoid tyramine-containing foods e.g. cheese, red wine and some fish as severe elevations in BP may occur.
Mechanism of Actions
Isoniazid is active against M tuberculosis, M bovis and some strains of M kansasii. One of its main mechanisms appears to be mycolic acid synthesis inhibition resulting in loss of acid-fastness and bacterial cell wall disruption.
Absorption: Absorbed readily from the GIT (oral) and after parenteral admin (IM); peak plasma concentrations after 1-2 hr (oral). Rate and extent may be reduced by the presence of food.
Distribution: Body tissues and fluids, CSF, crosses the placenta and enters breast milk.
Metabolism: Hepatic and enteral; acetylation by N-acetyltransferase to acetylisoniazid followed by hydrolysis to isonicotinic acid and monoacetylhydrazine, then conjugated with glycine to isonicotinyl glycine and monoacetylhydrazine is further acetylated to diacetylhydrazine.
Excretion: Via urine (as metabolites), via faeces (small amounts); 1-6 hr (elimination half-life), removed by dialysis.
Should be taken on an empty stomach. (Best taken on an empty stomach 1 hr before or 2 hr after meals. May be taken w/ meals to reduce GI discomfort.)
Storage Conditions
Oral: Store at 20-25°C.
ATC Classification
J04AC01 - isoniazid ; Belongs to the class of hydrazides. Used in the systemic treatment of tuberculosis.
Oral: Store at 20-25°C.
Available As
  • Isoniazid 0.1 gm
  • Isoniazid 100 mg
  • Isoniazid IP 100 mg
  • Isoniazid 150 mg
  • Isoniazid IP 150 mg
  • Isoniazid 2.33 mg
  • Isoniazid 2.33% w/w
  • Isoniazid 225 mg
  • Isoniazid IP 225 mg
  • Isoniazid 275 mg
  • Isoniazid 30 mg
  • Isoniazid IP 30 mg
  • Isoniazid 300 mg
  • Isoniazid IP 300 mg
  • Isoniazid 300 mg (4 tab)
  • Isoniazid 450 mg
  • Isoniazid 50 mg
  • Isoniazid 600 mg
  • Isoniazid IP 75 mg
  • Isoniazid 75 mg
  • Isoniazid 750 mg
  • Isoniazid IP 80 mg
  • Isoniazid 80 mg
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