Isozid (Isoniazid)

Isoniazid is used in the management of Mycobacterium tuberculosis infection and related disease states.

• Active tuberculosis: Reduces bacterial load and contributes to cure when combined with other antitubercular drugs.
• Latent tuberculosis infection: Prevents progression to active disease in individuals with a positive tuberculin test.
• TB prophylaxis in high‑risk contacts: Decreases the risk of developing active TB after exposure.

• After oral absorption, isoniazid is activated by the mycobacterial enzyme catalase‑peroxidase (KatG). The activated form inhibits the synthesis of mycolic acids, essential components of the mycobacterial cell wall, leading to cell wall disruption and bacterial death. This bactericidal activity is specific to actively replicating Mycobacterium tuberculosis. Because mycolic acids are vital for the integrity and impermeability of the bacterial envelope, their inhibition compromises the pathogen’s survival and enhances the efficacy of companion drugs in standard regimens.

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

Common Side Effects

• Nausea: mild gastrointestinal upset.
• Peripheral neuropathy: tingling or numbness in extremities.
• Hepatotoxicity: elevated liver enzymes and possible jaundice.
• Rash: mild skin irritation or itching.

Serious or Rare Side Effects

• Severe hepatitis: jaundice, dark urine, and liver failure.
• Anaphylactic reaction: sudden swelling, difficulty breathing, and shock.
• Sideroblastic anemia: impaired red blood cell production leading to fatigue.

Before prescribing isoniazid, clinicians should consider several safety precautions.

• Baseline liver function: assess hepatic enzymes prior to initiation.
• Alcohol consumption: avoid or limit alcohol to reduce hepatotoxic risk.
• Vitamin B6 supplementation: co‑administer pyridoxine to prevent neuropathy.
• Pregnancy and lactation: evaluate risk‑benefit as safety data are limited.
• Drug interactions: review concomitant medications for potential hepatic or neurotoxic effects.
• Store at room temperature: keep in original container, protect from moisture and heat.

Isoniazid interacts with several medicines, requiring careful management.

Major Interactions (Avoid)

• Rifampicin: reduces isoniazid plasma levels, compromising efficacy.
• Phenytoin: increases hepatic metabolism, lowering isoniazid concentrations.
• Carbamazepine: induces enzymes that diminish isoniazid effectiveness.

Moderate Interactions (Monitor Closely)

• Acetaminophen: combined hepatotoxic potential; monitor liver function.
• Alcohol: additive risk of liver injury; advise abstinence.
• Antiretroviral drugs (e.g., efavirenz): may alter isoniazid metabolism; adjust dosing as needed.