(eng; includes abstract) By Nierenberg AA, Cole JO, Glass L, The Journal Of Clinical Psychiatry [J Clin Psychiatry], ISSN: 0160-6689, 1992 Mar; Vol. 53 (3), pp. 83-5; PMID: 1548249; BACKGROUND: Fluoxetine is an effective serotonin reuptake inhibitor antidepressant that can fail to alleviate either insomnia or major depression in a substantial minority of depressed patients. Trazodone has been reported to be both an effective nonbenzodiazepine hypnotic for antidepressant-associated insomnia and a possible serotonergic antidepressant adjunct. We present a case series of patients with variable responses when trazodone was added to fluoxetine. METHOD: Eight consecutive depressed (DSM-III-R) patients taking fluoxetine were given trazodone either for sleep or as a possible antidepressant potentiator. RESULTS: Three (37.5%) of the eight patients (confidence interval = 4.0%-71.0%) had improvements in both sleep and depression. The remaining five patients either were unaffected by the addition of trazodone to fluoxetine or had intolerable adverse drug reactions. CONCLUSION: Trazodone may be an effective hypnotic and antidepressant potentiator when combined with fluoxetine for some patients, but its use may be limited by adverse effects.
Here's an even more interesting study:
When patients with TRD were teased out, the same trend was seen: responses in 1 (12.5%) of 9 patients in group 1, 5 (62.5%) of 8 in group 2, and 7 (70%) of 10 in group 3. Thus the combi-nation of a subtherapeutic dosage of trazodone plus pindolol was as effective as the same subtherapeutic dosage plus fluoxetine in the treatment of depressed patients and those with TRD, and both combinations were significantly better than trazodone alone.
The above is a very large read as it also concerns pindolol, but if you search for trazodone in the above URL you'll get the gist. Trazodone at 100mg was found to be effective in treatment-resistant depression when used together with fluoxetine.