The only risk I see with frequent use of dextromethorphan is what it could do to your brain. There is some evidence to suggest that NMDA antagonists such as DXM, ketamine and PCP may cause something known as Olneys Lesions though there are no human cases of this. Regardless of whether these drugs can caused damage to your brain or not they most certainly can affect your brain chemistry and put you into a psychosis or delusional state of mind. I have seen this first hand with DXM at high doses mutliple times a week and it could take months of non-use before you start to feel normal again.