My dog is on fluoxetine and trazodone. Is lethargy serotonin syndrome?

Serotonin syndrome is certainly possible with fluoxetine and trazodone together, or even just fluoxetine by itself, but what you're seeing with Luna does not sound like serotonin syndrome. With serotonin syndrome there's just too much of everything, too much energy, too much vomiting, too much diarrhea, too much muscle movement (tremors and seizures), too much pupil size. Sedation is an expected and desirable side effect of Trazodone, so in this situation I wouldn't worry. I would recommend giving a slightly smaller dose of trazodone tomorrow to see if that is enough to keep Luna calm. And always, if you're worrying about it and not sure, take Luna to an emergency clinic for an quick check in. Also, these two drugs are used together VERY commonly in veterinary medicine, but I don't think human doctors use them together, so you'll see lots of precautions against it while reading online.

I hope that helps you feel better! Thanks for using Petco Pet Education Center, formerly Petcoach.