Yeah it seems that citalopram is only a weak CYP2D6 inhibitor and shouldn't cause any significant difference in its activity or the metabolism of DXM. There is only one other thing to consider that I can think of: serotonin syndrome. DXM, along with citalopram, is a serotonin reuptake inhibitor and the two combined could possibly elevate the levels of serotonin in synapses to a toxic degree. This might cause hyperthermia, tremor, dysphoria, tachycardia, and nausea.
That said, who knows what your individual reaction might be but it seems at least slightly risky. If I were you and I were completely set on trying this, I would at least try the combination at a lower dose of DXM first to test its effects on you. There are some reports of experiences like this (in fact, one on erowid with 20 mg citalopram and 300 mg dxm with chlorpheniramine) and that one at least ended up alright. [For the record, I'd VERY strongly suggest keeping away from DXM with anything else in it including chlorpheniramine, diphenhydramine, or phenylephrine, being that at least the first two are serotonin reuptake inhibitors and are both 2D6 inhibitors, which would increase the likelihood, duration, and danger of serotonin toxicities] Definitely at least take it easy with this combination as serotonin syndrome has been reported and it is 1) incredibly unpleasant and scary and 2) potentially lethal. Anyways, good luck!
-Jaguraguguru
