First, I would recommend doing some research on DXM. Go to Erowid or Dextroverse, which both have good information. I have done a fair amount of drugs in my lifetime, and DXM is one that can be very powerful. If you take something that has something other than pure DXM in it, it has the potential to kill you. I'm not trying to scare you off, but giving you the facts. Before you go drinking just any cough syrup, make sure you know all the ingredients.

If after doing research you decide you still want to do it, start with small doses (150-200 mg at most). Even if nothing happens, don't take more. DXM has a way of sneaking up on you, and usually at the minimum it takes several hours for the trip to reach the full potential. Sometimes you won't feel anything at all until 60 to 90 minutes after ingestion. There is no coming back once you've done too much, so it is much better to go slow and be safe versus doing too much too fast and having a really bad or dangerous trip.