If this is your first time doing DXM, make sure to get only DXM-containing products, such as Delsym or robitussun gels/similar brand, but based on your dosed it seems you have a syrup in mind.
please pick something else. the syrups are dangerous, and they have other ingredients in them. You don't want anything
definitenly with acetaminophen
OR chlorpheniramine in it.
The above two
can and will kill you. you also want to avoid phenylephrine, guafienesin, doxylamine, and diphenhydramine.
If it is your first time. Just get a 3oz bottle of delsym and look up how to do a water extraction (all you need is a soda pop bottle and a few hours) and then you have some pure dextromethorphan polistirex powder.
And it just tastes like sand!

No unadulterated nastiness either.
About the drinking the next day, to answer your question.
no it's not risky, per se, as in it is not going to kill you, but it is not certainly not good for your health, or for your liver, as Speed King said - as dextromethorphan needs to be heavily hepatically metabolized and processed before it can be utilized and become active.
^kleinerkiffer, wouldn't going off of DXO's half-life be better, since that's the active drug? And DXM is really just the prodrug