Grey stool on antibiotics? waiting for doc to call back. is this life threatening?

Change in bowels: The amount, color, frequency of stool varies with foods, speed of gut emptying, medications/supplements ingested, gut flora (can be altered by your antibiotics), additives (like blood), subtractions (like absent bile), infection, inflammation, ; other factors. If stool pattern has changed, is unusual for you, ; the change persists, or is accompanied by other symptoms, await your doctor's advice.