The ingredients of food are "metabolized" in the liver that means split to a large extent into the same chemically.  Salicylic acid (in normal amounts) such as those exist in the (normal) food portions, is reduced to a small quantity and thus not reaches the blood circulation. Therefore Guaifenesin users also can consume food safely. The situation is different for concentrates and extracts (juice, candy, licorice, tea, etc.), because there the salicylate is much higher.