Yes, only buy Robitussin Maximum Strength Cough. That lists DXM as it's only active ingredient. I don't know about the long term effects of DXM alone, but I do know that the other active ingredients found in other cough meds (besides DXM) can hurt you even the first time you do it.

I've only done DXM a few times, and I'd imagine most ppl do DXM just casually, not as a regular thing. Or maybe there are ppl who do it everyday, I dunno. In either case, I've never heard anybody complain of liver problems or whatever just from doing DXM. But you might wanna check out www.erowid.org for info on long term effects of DXM.