In Mastering the Core Teachings of the Buddha: An Unusually Hardcore Dharma Book, Daniel Ingram writes about how meditation can be destabilizing.
Just as serious athletes can hurt their bodies when they take a misstep or push themselves beyond their limits, just so serious mental athletes can strain their minds, brains, and nervous systems, and strained brains can sometimes function in very strange ways. To rewrite the operating system rapidly while it is running doesn’t always go so well in the short term or occasionally in the long term. Thus, while I will include nearly endless exhortations to find the depths of power and clarity that you are capable of, I will also add numerous warnings about how to keep from frying yourself.
By “frying yourself”, I mean explicitly severe mood instability and psychotic episodes, as well as other odd biological and energetic disturbances, with some practitioners occasionally ending up in inpatient psychiatric facilities for various periods of time. Exactly how much of this is nature (their own “inherent wiring” and potential for mental pathology), how much of it is nurture (practicing hardcore meditation techniques in high doses such as those presented here), and how much is related to other unidentified factors is a question that is still being worked out, just so that you are not in any way uninformed about the still-developing state of modern science as it applies to the art of intensive meditation.
―Mastering the Core Teachings of the Buddha by Daniel Ingram
I am aware of a meditator for whom awakening gave him bipolar disorder. Bipolar disorder is a life-wrecking mental illness. Since awakening via Zen, this man has been in inpatient psychiatric wards three times for psychotic (manic) episodes. He will have to be on medication for the rest of his life. This meditation has awful side effects. The meditator’s family has a history of bipolar disorder. DO NOT PURSUE AWAKENING IF YOU HAVE A FAMILY HISTORY OF BIPOLAR DISORDER. The same goes for if you are a slightly manic person in general, since meditation can remove the breaks on mania.
Interestingly, the man in question reached awakening after an unusually low number of hours on the cushion, possibly less than 100. This might not be a coincidence.