I read a recent review last week which warned against the use of quetiapine for treating non-psychotic mood disorders.
Yet, I believe there are a number of reasons to consider quetiapine and similar medications for non-psychotic states:
1) there is a much lower risk of the medication causing mania or psychosis. With antidepressants, there is always the risk of mania induction. Quetiapine not only would not cause mania, it could protect against it.
2) the use of quetiapine could reduce the likelihood of other sedatives, such as benzodiazepines, being used as often. Benzodiazepine dependence is very common. Quetiapine is less "addictive."
3) the doses of quetiapine in non-psychotic states can often be very low (under 100 mg) causing a much lower risk of metabolic side-effects than full doses of 400-600 mg per day or more.
What about research evidence?
Mezhebovsky et al (2013) published results of a multi-centre study involving about 450 elderly patients, showing that quetiapine 50-300 mg (mean = 168 mg) daily for 11 weeks, led to significant improvements in generalized anxiety symptoms, compared to placebo.
( http://www.ncbi.nlm.nih.gov/pubmed/23070803 )
As with most effective treatments, the medication group had about twice as much improvement as the placebo group. It is true that sleep improvement could account for a significant proportion of the overall symptom score improvement, but there was also improvement in the other symptom domains. There were no major metabolic side effect problems in the quetiapine group. The most common side effect was somnolence (sleepiness).
A 2012 review by Sanford and Keating ( http://www.ncbi.nlm.nih.gov/pubmed/22519923 ) showed an abundance of evidence that quetiapine is beneficial for treating bipolar depression (typically at doses of 300 mg/day) and for preventing recurrences of any mood episode. For those who benefit acutely from quetiapine, there is evidence that it is a more effective mood stabilizer--on its own--than lithium.
In unipolar depression, quetiapine would be most commonly used when a standard treatment such as an antidepressant was not working well. In a study by El-Khalili et al (2010), quetiapine up to 300 mg per day was added as an adjunct to previous therapy for non-remitting depression:
( http://www.ncbi.nlm.nih.gov/pubmed/20175941). They showed a modest benefit of adding the quetiapine, particularly at a higher dose of 300 mg/d. A nice component of this article is the inclusion of symptom subtypes. Many critics would argue that quetiapine might simply be sedating, and improve sleep, leading to most of its benefit over placebo. These results confirm that quetiapine improves sleep symptoms. But there were also symptom improvements in other categories, such as pessimism, inner tension, and concentration impairment.
In conclusion, I think that quetiapine deserves to be considered as a medication option for non-psychotic conditions. In many cases, there are comorbidities or diagnostic uncertainties, in cases of depression. Many studies exclude patients who have comorbidities, or who do not neatly fit into diagnostic categories. Quetiapine is unlikely to worsen comorbid conditions, and may be beneficial for many. This makes it a safe option to think about if there is uncertainty or complexity in the diagnosis. Standard antidepressants in this situation may carry a higher risk of causing new problems, including agitation or a manic state.
The risks of metabolic side effects, etc. need to be watched for carefully, with consideration of stopping or changing the plan if problems of this type arise.