What is a major contraindication for INH therapy?

A major contraindication for isoniazid (INH) therapy is active liver disease. This is because INH is metabolized in the liver, and in patients with active liver disease, there is a risk of significant hepatotoxicity. The liver's ability to process medications can be severely impaired in these patients, increasing the likelihood of adverse effects and potentially leading to liver failure. Therefore, if a patient has active liver disease, healthcare providers will typically avoid prescribing INH or use extreme caution, closely monitoring liver function if the drug must be used.

Other conditions listed, such as diabetes mellitus, pregnancy, and severe hypertension, may require careful management and consideration but do not outright contraindicate the use of INH. For example, in diabetic patients, INH can still be prescribed but requires monitoring of blood glucose levels. During pregnancy, although there are potential risks, INH may be considered if the benefits outweigh the risks. Similarly, severe hypertension is not a direct contraindication but would require managing the patient's overall health before initiating therapy.