Guaifenesin and active ingredient

When my wife takes an OTC medication containing guaifenesin, she experiences sharp stomach pains that almost totally incapacitate her for a number of minutes. She also experiences dizziness and an intense desire to "either throw up or poop," in her words. We've known for a while that Mucinex has this effect on her. It wasn't until today, when she took some Robitussin CF and experienced the same side effects, that we looked up the name of the active ingredient. Now we know that she needs to avoid guaifenesin.