What is the primary side effect of Isoniazid?

The primary side effect of Isoniazid is peripheral neuritis, which is specifically due to its role in inhibiting the synthesis of pyridoxine (vitamin B6). Isoniazid can lead to a deficiency of this vitamin, which is essential for proper nerve function. This deficiency manifests as numbness, tingling, or pain in the hands and feet—an indication of nerve damage, often referred to as peripheral neuritis.

While hepatotoxicity is a significant concern with Isoniazid use, especially in certain high-risk populations, it is not typically classified as the primary or most immediate side effect observed with this medication. Nausea and rash can also occur, but they are not as characteristic or prevalent compared to the risk of peripheral neuritis associated with Isoniazid, particularly in cases where patients may not receive adequate vitamin B6 supplementation.