Can you overdose from dextromethorphan? Question about robotripping and otc meds.?

I know dextromethorphan is the ingredient that makes you "robotrip". How much pure dextromethorphan would it take for me to "robotrip"? What is the harmful ingredient in cough meds that can hurt you? I have two different type of cold medicines and the only active ingredient they share is dextromethorphan would it be okay if I took these two medicines together? Would I feel high or anything?