I looked up the robitussin on the internet. And this what I found. Plus some more methods not to follow.
The following methods will not, and we repeat, will not help you get pregnant.
Taking robitussin® cough syrup before trying to conceive.
Taking the herb wild yam.
Not urinating after sex.
Lying with your feet over your head after sex.
Standing on your head after sex.
Making love during the day with the lights on.
Having sex every day instead of every other day.
Sitting in the same chair a pregnant woman has sat in.
Staring into the eyes of a pregnant woman.
Putting ice packs on your partner’s testicles. (ouch!)
but I couldn't find the side effects of it. But I guess it is just like everything else that you are not suppose to take while pregnant.