Isoniazid is used alone or with other drugs to treat tuberculosis (TB) and to prevent it in people who have had contact with tuberculosis bacteria. It eliminates only active (growing) bacteria. Since the bacteria may exist in a resting (nongrowing) state for long periods, therapy with isoniazid (and other antituberculosis drugs) must be continued for a long time (usually 6-12 months).
Isoniazid may cause side effects. Tell your doctor if any of these symptoms are severe or do not go away: diarrhea and vision problems
If you experience any of the following symptoms or those listed in the IMPORTANT WARNING section, call your doctor immediately: eye pain, numbness or tingling in the hands and feet, skin rash, fever, swollen glands, sore throat, unusual bleeding or bruising, stomach pains or tenderness.