You are as sick as a dog when you are very sick with a cold, flu, or stomach problem. Example: "I don't know what I ate this morning, but I have been as sick as a dog all day."

As sick as a dog emphasizes how very sick you are. Example: "I heard you were uncomfortable yesterday." Reply: "Uncomfortable? I was as sick as a dog!"

You are much too sick to do the things a normal person does when you are as sick as a dog. Example: "Sorry, but I am not going to be able to come in to work today. I'm as sick as a dog."