i take my previous statement back, you need to get the regular Robitussin, not the DM. The regular Robitussin has only the Guaifenesin as the active ingredient...you're has the Dextromethorphan Hydrobromide in it too, and even though its only 10mg, it can make what you're trying to achieve ineffective (at least thats what i understand when i just looked it up again)
sorry and hope this helps!
kaycie