Isoniazid Side Effects

Some side effects of isoniazid may not be reported. Always consult your doctor or healthcare specialist for medical advice. You may also report side effects to the FDA.

For the Consumer

Applies to isoniazid: oral syrup, oral tablet

If you experience any of the following serious side effects, stop taking isoniazid and seek emergency medical attention or contact your doctor immediately:

• an allergic reaction (difficulty breathing; closing of the throat; swelling of the lips, tongue, or face; or hives);

• unusual weakness or fatigue;

• nausea, vomiting, or loss of appetite;

• abdominal pain;

• yellow skin or eyes;

• dark urine;

• numbness or tingling in your hands or feet;

• seizures;

• blurred vision; or

• confusion or abnormal behavior.

Side effects other than those listed here may also occur. Talk to your doctor about any side effect that seems unusual or that is especially bothersome.

For Healthcare Professionals

Applies to isoniazid: intramuscular solution, oral syrup, oral tablet

Nervous system

Overdose of isoniazid has been associated with uncontrollable seizures. Dialysis may be required to decrease isoniazid blood levels, thereby controlling seizures. Seizures, lethargy, and confusion have also been reported in patients with chronic renal failure. Other patients at risk for neurotoxicity include the malnourished and alcoholics. Optic neuritis has also been reported in patients on hemodialysis.

Peripheral neuropathy has been observed and occurs frequently, especially at doses greater than 300 mg daily. Neuropathy may be prevented or attenuated by coadministration of pyridoxine 50 to 100 mg daily. Other neurologic reactions, although rare, have included visual disturbances, ataxia, and seizures.

Hepatic

Hepatitis has been reported in less than 5% of patients receiving isoniazid alone. Jaundice is usually preceded by a prodromal illness with fatigue, nausea, malaise, abdominal pain, and anorexia. Asymptomatic increases in liver function tests may occur. Isoniazid should be discontinued if hepatotoxicity occurs, usually defined as SGOT greater than four times normal.

The mechanism of hepatic injury is unknown but may be related to the acetyl metabolite of isoniazid. Patients exhibiting hepatotoxicity are more likely to be fast acetylators of isoniazid. Eight cases of severe hepatitis resulting in death or transplantation have been evaluated by the Department of Health of New York. Duration of isoniazid use before onset of hepatitis ranged from 21 to 142 days, and seven patients continued use of isoniazid at least 10 days after onset of symptoms. Massive hepatic necrosis was a common finding and cholestasis was present in two of five cases.

The risk is age related with a greater occurrence reported in patients who are 35 years or older. The risk of hepatitis is also increased in patients who consume alcohol daily, in women, and in minorities. In a study of 2651 women beginning isoniazid preventive therapy during pregnancy or postpartum, 5 cases of isoniazid-induced hepatitis were identified, including two fatalities. In another review of deaths due to isoniazid, eight of 21 women between 15 and 44 years old were within one year postpartum. In general, death due to isoniazid hepatotoxicity occurs more frequently in women than men.

Fulminate hepatitis, characterized by jaundice, disorders of consciousness and elevated serum transaminases up to 80 times the upper limit of normal, has occasionally occurred in patients receiving isoniazid with rifampin. Rifampin, by virtue of its enzyme-inducing activity, likely increases the reactive metabolite of isoniazid thought to be responsible for the hepatotoxicity associated with isoniazid.

Monthly monitoring and interviewing of patients should take place. Patients should be fully informed regarding the risk of hepatotoxicity associated with isoniazid, educated about the signs and symptoms of liver damage, and instructed to contact their physician immediately if they develop signs or symptoms.

Hematologic

Several cases have been reported of pure red cell aplasia attributable to isoniazid. Abnormalities resolved following drug discontinuation.

Hematologic abnormalities such as anemia have been reported. Anemia is generally reversible following discontinuation of isoniazid. Agranulocytosis, thrombocytopenia, and eosinophilia have rarely been reported.

Hypersensitivity

Hypersensitivity reactions including drug fever, rash, lymphadenopathy, vasculitis, and urticaria have been reported but are rare. These reactions generally subside following drug discontinuation.

Immunologic

Isoniazid-induced lupus-like reactions have been reported with an incidence of approximately 1%. However, as many as 22% of patients on this drug may develop positive antinuclear antibodies. Drug discontinuation is recommended if a lupus-like reaction occurs.

Psychiatric

Psychosis, depression, and aggression have been rarely reported with isoniazid therapy. Some patients with preexisting schizophrenia have experienced exacerbations when isoniazid was started.

Gastrointestinal

Gastrointestinal adverse effects have included nausea, vomiting, and epigastric distress. A few cases of pancreatitis have been reported.

Metabolic

Metabolic side effects such as pyridoxine deficiency and pellagra have been reported. Isoniazid induced hypocalcemia and hypophosphatemia has been observed and may be due to altered vitamin D metabolism.

Local

Local irritation has been observed at the site of intramuscular injection of isoniazid.

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