You shouldn't take Dextromethorphan just to "get high." It is a very powerful drug and you can really do some psychological damage to yourself if you aren't experience with psychoactives or have emotional problems etc. Bad trips don't usually occur with smaller doses, but it is always possible.

Don't take very much your first time. A certain percentage of the population has an enzyme deficiency which makes Dextromethorphan especially dangerous. Read up on dosages on Erowid. You probably want to go for a low 2nd plateau if at all.

Only take cough syrup that has Dextromethorphan as the ONLY active ingredient. I can't stress that enough.

Keep safe.