Hi, Mucinex is used to treat symptoms of common cold and stuffy nose, it contains guaifenecin and pseudoephedrine that are expectorant and decongestant respectively. It is known to cause side effects of anxiety and nervousness, but not that begins after many weeks of ingesting the drug for just a day. There might be another stressor that triggered the illness or it may have happened spontaneously depending on the vulnerability the patient had. Either ways, the next step would be to evaluate and treat the likely severe depressive illness with suicidality by a psychiatrist at the earliest without any further delay. She needs immediate medical attention. Hope this helps, Dr Anjana rao. Hope this helps,