There’s evidence that SSRIs (but not tricyclic antidepressants) are useful in the treatment of negative symptoms of schizophrenia. Many of the studies were done with fluvoxamine but there have been studies with other drugs such as mirtazapine. Paradoxically, the use of an SSRI, which increases 5HT, with clozapine, which inhibits 5HT, has shown clinical effectiveness. The improvement is present when patients have been screened for co-morbid major depression and excluded from the studies so the effect on negative symptoms is thought to be independent of its antidepressant effect. There’s also evidence that the addition of an antidepressant can help with cognitive deficits in schizophrenia. Some literature suggests that the dose of the SSRI can be lower than that for depression but did not suggest a therapeutic range. It’s likely, given the equivalence in efficacy between SSRIs, that they’re all effective but this hasn’t yet been determined.