^^ I'm afraid do not know the biological functioning of this, perhaps something to do with the CYP2D6 and glucose-insulin metabolism. I can only report anecdotally from personal experience. IME, consuming sugar enhances the effects, and as there is no sucrose/glucose syrup in diabetic tussin, I suspect this is why I've found it to be less effective, (although, yes it does taste better) unless I take it with candy, sugary soda or sweets and such. If anyone knows why this is, I'd greatly like to know.
As for the laxative effect, its most likely due to sorbitol, a common ingredient in tussin. The effect is caused by increased water being retined in the intestines. Eating rice can greatly help absorb this excess water.