Have you ever looked closely at the ingredients listed on a cough syrup label? It's poly-this & prop-that along with dyes, high fructose corn syrup, & a ton of modified sugars. Used in soap making & other commercial uses, I noticed that even glycerin is even listed under inactive ingredients in Robitussin DM (although there are objectors, its generally regarded as safe for internal use by the FDA- is even used as a sweetener- but as a humecant I'm not sure it makes sense to have it in a product that is used to dry up mucus). Regardless, fillers destroy our natural occurring enzymes & good bacteria in our digestive tract- where 80% of our immune system is located! Not something I'd like to damage, especially if you're trying to heal from an illness.
But what do you do when you have a bad cough & need relief fast? First, try to avoid products with lots or sugar, as we all know that sugar increases mucus production & will actually make your cough worse- very counter productive! Secondly, sugar should be avoided during an infection because you are essentially feeding the bacteria & encouraging them to reproduce- instead, eliminate the factors that encourage their growth & your body will heal more quickly as you make the environment more inhospitable to foreign bodies.