If the DM stands for dextromethorphan (an expectorant) it would fit. They warn people on SSRIs to avoid it because of interactions - has some affect on serotonin. It's also usually included in things like Dayquil.
Dextromethorphan is a "dissociative anesthetic" that some people use recreationally. It's a bit of a cliche that kids will sometimes abuse cough syrup because it gives them a mild ketamine-like high. I suppose it's not shocking that it could potentially have a negative psychological interaction.
It looks like it has SSRI-like effects itself. So it also shouldn't be taken by people on older anti-depressants (MAOIs) as it could cause 'serotonin-syndrome' (which can be fatal if not treated quickly).