Earlier this month JAMA Psychiatry published a consensus statement on ketamine for mood disorders (such as depression and bipolar depression) jointly written by a number of physicians from prominent academic centers across the United States. While somewhat conservative in it's interpretation of available data, it is notable that academics who have been historically opposed to the clinical use of ketamine, seem to be gradually coming around and while not concretely endorsing its use, do seem to agree on ketamine being a viable treatment option. As described in the article, plenty of data exists to support the short-term use of ketamine for depression, but the field is still developing in terms of high quality randomized controlled trials exploring ongoing maintenance treatment. This is not uncommon in psychiatry--that medication treatments are studied for shorter periods of time--though as the data accumulates in favor of ketamine we will eventually see more such long-term studies. Until that time, clinical experience, case studies, open label trials, and related treatments (such as maintenance ECT) guide the use of ketamine for maintenance of mood disorders.
JAMA Psychiatry article: http://jamanetwork.com/journals/jamapsychiatry/article-abstract/2605202