My bun was 11 in january. now it dropped to 5 which is low. what can cause 1) low bun 2) such a drastic drop in 10 months? please tell all possible causes. i will ask my dr but other opinions are helpful. all other results from the cmp were normal?

Causes of a low BUN include a diet poor in protein,malnutrition,drinking too much water and severe liver damage. In your case I wouldn't put any significance on it as I doubt that any of these factors are present.

All laboratory results need to be interpreted in the clinical context and the doctor who ordered the tests is usually in the best position to do that. Talk to the doctor who ordered the tests. Having said that, lower BUN is likely due to your being better hydrated. It is not indication of kidney disease. If your CMP is normal, I would not be concerned. Wish you good health!