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Side effects occur in approximately 3.4 percent of persons. The most common side effects are diarrhea, nausea, vomiting, light sensitivity, vision problems. INH may also cause severe liver damage. If one experiences any eye pain, numbness or tingling in the limbs, rash, fever, swollen glands, sore throat, bleeding, excessive bruising, or stomach pains this may be early indications of liver damage.
INH causes "clinically significant" liver damage or hepatic injury in 1 percent of patients and abnormal enzyme levels in 10 to 20 percent of patients. Studies have shown that INH can induce apoptosis by disrupting mitochondrial membrane potential and breaks in DNA strands.
In addition, isoniazid also interferes with niacin metabolism sometimes resulting in nervous system toxicities. Peripheral neuropathy is the most common side effect, but toxic encephalopathy, optic neuritis, cerebellar ataxia, and mild psychoses are also observed.
Sometimes people hypersensitive to isoniazid show symptoms of fever, pruritus, and rashes. In rare cases it can result in arthralgias, anemia, or agrunulocytosis.