Which of the following is a side effect to monitor when starting INH?

When starting isoniazid (INH), gastrointestinal upset is indeed a notable side effect to monitor. This medication, commonly used for the treatment and prevention of tuberculosis, can lead to symptoms such as nausea, vomiting, and abdominal pain. Monitoring for gastrointestinal upset is essential because it can affect the patient’s adherence to the treatment regimen, leading to incomplete therapy and potential development of drug resistance.

While other side effects, such as visual disturbances, fatigue and lethargy, and neurological symptoms, are also possible with the use of INH, gastrointestinal upset is one of the more common and immediate concerns seen at the outset of treatment. It's crucial for healthcare providers to be vigilant about these symptoms to ensure that the patient can continue their treatment effectively.