I would warn against this combination at all costs. Have I taken DXM while on serotonergic drugs, such as SSRI's? Yes, I have, and I have survived, but I have had a few experiences where I knew my body was being assaulted with the barrage of serotonergic effects, and it was not like a simple DXM trip without any serotonergics; it was toxic-feeling - not pleasant. And, I am not talking about large doses of DXM with SSRI's and similar drugs, but rather doses not much higher than the recommended therapeutic doses to suppress cough. Serotonin syndrome can be quite mild, or very severe and life-threatening, or anywhere in between. I know I have experienced very mild bouts of serotonin syndrome, and even that feels very wrong and very unhealthful. I would advise, most strongly, against combining any dose of an SSRI or similar serotonergic drug with any dose of DXM. I am fairly sure even my 'very mild' experiences put my body at significant risk. There will be some interaction, and it is just a very, very dangerous game to play. Also, it should be remembered that different individuals respond very differently to a given dose of DXM; some people can have a fairly risky reaction, even if not on any other drugs, after having taken only 30 mg or so of DXM, because of certain variances in how one person metabolizes DXM to the next, and because of the fact that the chemical response to DXM (and all serotonergics) varies a great deal from one person to the next. When I did combine DXM with a serotonergic anti-depressant, I was not in a good mindset, and was a bit passively suicidal (if you might call it that), and even then restricted my DXM dose to between 30 mg and 90 mg, with very notable psychotropic effects, and keep in mind, I am a normal metabolizer of DXM and tolerate serotonergic drugs quite well. Small doses of DXM, in those combinations, really caused marked psychological and physiological effects on my mind and body; I remember I'd always be burning up - feeling hot as hell - and sweating beyond profusely, I'd have saucer-sized pupils (much more dilated than after having taken a respectable dose of amphetamine, including MDMA), I would feel very, very queer, like I was thrust into some plateau sigma without any of the desirable effects of such a plateau, I would have trouble talking, I would be fairly confused over everything, experience grave dysphoria, have my heart rate very well above what is considered tachycardia, I'd just be praying for like a Thorazine to help even some of it out, and I'd experience countless other horrible effects as a result. Now, DXM alone, without any serotonergic drugs, had always been fine, even at markedly higher doses than the above mentioned 30 mg to 90 mg (well above 300 mg), with few side-effects and transient, if any, dysphoric experiences. So, I'd really, really take serotonin syndrome VERY SERIOUSLY; always, be cautious - you can always wait until a certain serotonergic drug has been discontinued for a while to have fun with DXM, but dropping Zoloft for just a few days to use DXM is just not safe. I beg you heed this warning.
AND, even if you've been in the clear with the same or higher doses, before, do not risk falling into serotonin syndrome - it really does only take one time, regardless of getting through past experiences okay. Even ultra-light use, I would warn against with all my might. Please, just avoid the DXM. It is not worth the very real, very big risk of significantly endangering your life.
