Coriciden actually has an anti-histamine in it called CPM. In high doses it causes anti-cholinergic effects. It's not fun. It'll make you completely delerious, you'll think things are there when they're really not (this is in no way like psychedelic visuals, and isn't enjoyable).
High doses of CPM and DXM together are dangerous.
If for whatever reason you can't find pure DXM powder, then I'd suggest extracting DXM from Robitussin Maximum Strength Cough. Your yield wont be 100%, but the body load and negative physical effects are greatly reduced (esp. compared to downing 40 Robo gels).
There's quite a few teks online for the extractions. It's quite simple, but don't use a plastic bag for a separatory funnel. Non-polar solvents like Naptha dissolve plastics.
If you are not willing to do an extraction either get Dex Alone. I've found it in the stores ONCE. It's not exactly an easy product to come across.
If you're ok with drinking cough syrup then get Robitussin Maximum Strength Cough (not Cough & Cold! They're almost identical looking, make sure to look carefully). The reason I recommend this for both the extractions and otherwise is because it's relatively easy to find, has no other actives besides DXM, and has the highest concentration of DXM (one bottle has 356mg DXM HBr).
Those Robitussin Cough Gels (Robo gels) are ok, but I always seem to have a pretty bad case of nausea throughout the entire experience with these, which usually isn't the case with DXM in other forms.