Well, everyone I got into it now takes the pills, but I'm old school. I used to drink the syrup all the time.
However, it's been awhile since I've done it, but the last times I did it I extracted the DXM from the cough syrup, so as not to have to deal with the high amount of sugar which puts a lot of stress on the body, and the disgusting, thick syrup.
I've gone on DXM binges in the past and I've never noticed any changes. Took IQ tests (legit, with a psych) way before, shortly before, and after having done it even my first time. To this day, my IQ still remains the same, and I have no problems with my verbal or motor skills. Cognitive function is fine, too.
I love my DXM.