Hi, I appreciate your concern for your beloved husband. It is not safe to give Robitussin Nighttime Cough DM while on Quetiapine. Quetiapine is an atypical antipsychotic used in the treatment of schizophrenia or bipolar disorder. Taking Robitussin Nighttime Cough DM together with Quetiapine may increase side effects such as drowsiness, blurred vision, dry mouth, heat intolerance, flushing, decreased sweating, difficulty urinating, abdominal cramping, constipation, irregular heartbeat, confusion, and memory problems. These side effects may be more likely to occur in the elderly or those with a debilitating condition. So avoid giving Robitussin Nighttime Cough DM together along with Quetiapine. For his cough he should be examined by a doctor to rule out pneumonia or other causes. However for the symptomatic relief of cough he may be given. Guaifenesin which is safe and not known to cause any harmful interaction if taken along with Quetiapine. Hope these answer your query. Let me know if I can assist you further. Wishing your husband a very good health. Kindly rate these answer if you are convinced. Regards, Dr. Mohammed Taher Ali