well taking DXM in low doses (50-100mg should be plenty) doesnt seem to have much effect on respiration, and I know this because the first 4 times I used DXM, I didn't take enough (highest dose i took was 150mg) and I remember testing my breathing to see if it was altered, which I always check for when trying new drugs, and feeling increased ease with breathing, and for a minute i was expecting to feel a stimulant drug, which was more or less how it was described to me ("its sorta like E, dude"). Many people seem to use DXM with opiates to potentiate them, and So have I, and I felt the full range of euphoria and nod, but I didn't feel any significant changes in breathing. this does not necessarily mean, however, that you will have the same experience, but its quite probable that you wont encounter any adversity, assuming you havent dosed too high with either substance.
And I'm actually not sure whether you have to take them simulataneously to reduce tolerance, but from what I've read, it seems to make more sense that you do have to take them at the same time.
