Can my 45 lb 4 yr old take children's mucinex (guaifenesin) cough and similasan kids cold and mucus relief together?

Yes: mucinex (guaifenesin) is guaifenisin and the similisan is homeopathic (no pharmacologic substances and the homoepathic substances are in tiny doses) no interactions

Why?: Mucinex (guaifenesin) is a standard expectorant (loosens mucus) and can relieve symptoms. Similasan is a homeopathic agent. The biggest risk is not knowing if one or the other helps your son. Most people who believe in homeopathy would avoid standard medications. I personally do not believe that homeopathy offers any benefit.