Navratna Ayurvedic Cool Hair Oil is intended for external use only. Applying a small amount externally on the neck is generally not expected to affect kidney function, since it is not ingested. However, as you have chronic kidney disease and cervical spondylosis, it is best to consult your doctor before using any topical product regularly, especially if you have sensitive skin or allergies. If you notice irritation, redness, itching, or discomfort after application, discontinue use and seek medical advice.