Sick as a dog and Sick as dog

Sick as dog and sick as a dog are semantically related In some cases you can use "Sick as dog" instead an adjective phrase "Sick as a dog".

Sick as a dog and sick as dog are semantically related in sick topic. You can use "Sick as a dog" instead an adjective phrase "Sick as dog", if it concerns topics such as not feeling well, not healthy, not comfortable.