Nidrazid (Isoniazid)

• Isoniazid is an antitubercular agent used in the management of Mycobacterium tuberculosis infection.
• Active tuberculosis: contributes to bacterial killing as part of combination therapy, leading to cure.
• Latent TB infection: reduces risk of progression to active disease when given as preventive therapy.
• Post‑exposure prophylaxis: administered to recent contacts of TB cases to prevent infection establishment.
• Tuberculosis in children: used as part of pediatric treatment regimens to achieve outcomes comparable to adult therapy.

• Isoniazid is a pro‑drug that, after intracellular activation by the mycobacterial enzyme KatG, forms a reactive complex that inhibits the synthesis of mycolic acids, essential components of the mycobacterial cell wall. The disruption of mycolic acid production compromises cell wall integrity, leading to bacterial death, particularly in actively dividing bacilli. The drug penetrates macrophages where Mycobacterium tuberculosis resides, ensuring intracellular activity.

Isoniazid may cause a range of adverse reactions, varying from mild to severe.

Common Side Effects

• Nausea: mild stomach upset often occurring after the first dose.
• Vomiting: occasional emesis, usually transient.
• Loss of appetite: reduced desire to eat during early therapy.
• Peripheral neuropathy: tingling or numbness in extremities.
• Rash: skin redness or mild eruption.
• Fever: low‑grade temperature elevation.

Serious or Rare Side Effects

• Hepatotoxicity: elevated liver enzymes and possible liver injury.
• Severe hypersensitivity reactions: including Stevens‑Johnson syndrome.
• Drug‑induced lupus erythematosus: autoimmune manifestations.
• Seizures: rare central nervous system events, especially in overdose.
• Acute liver failure: life‑threatening hepatic dysfunction.
• Anemia: reduction in red blood cell count.

Before initiating therapy, several safety measures should be observed.

• Baseline liver function testing: assess hepatic enzymes before starting treatment.
• Monitor hepatic function: repeat tests periodically during therapy.
• Provide pyridoxine supplementation: reduce risk of peripheral neuropathy.
• Avoid alcohol: concurrent use increases hepatotoxic risk.
• Adjust dose in renal impairment: use caution in patients with reduced clearance.
• Store at room temperature: keep in the original container, protect from moisture.

Isoniazid interacts with several medicines; some require avoidance while others need close monitoring.

Major Interactions (Avoid)

• Rifampicin: induces hepatic enzymes, markedly lowering isoniazid concentrations.
• Pyrazinamide: additive hepatotoxicity when combined in TB regimens.
• Antiretroviral drugs (e.g., zidovudine): may increase toxicity and require dose adjustment.

Moderate Interactions (Monitor Closely)

• Warfarin: isoniazid can enhance anticoagulant effect, necessitating INR monitoring.
• Carbamazepine: enzyme induction may reduce isoniazid levels.
• Phenytoin: similar enzyme‑inducing effect, potentially lowering efficacy.
• Acetaminophen: combined hepatic load warrants liver function surveillance.