Common brands: Delsym 12 hour, Robafen Cough, Tussin Cough (DM only)
https://en.wikipedia.org/wiki/Dextromethorphan
I found out most DM liquid cough medicines contain this ingredient, including Amazon’s own, Rite-Aid, Robitussin (actually most Tussin DM’s. Lot of them have this ingredient. I went on Amazon to check this out.