Can taking an expectorant for your cold cause restlessness or hyperactivity?

Maybe: It is much more likely to be caused by a combo pill that has the expectorant and cough suppressant. Dextromethorphan is the ingredient in otc cough meds used to calm a cough. Some people are sensitive to it at recommended doses but at high doses or in combo with other otc or rx medications it can cause many unpleasant and potentially dangerous side effects in anyone who uses it. Check labels.