Here’s an interesting conversation between Brown University neuroscientist (and meditation teacher) Willoughby Britton and yoga and Buddhism teacher Michael Stone. Britton, as a good scientist, is interested in cataloguing the confusing, unpleasant, and sometimes harmful effects that meditators may experience, including cognitive and sensory aberrations, emotional difficulties or challenges, changes sense of self, and disturbing physiological manifestations.
My experience is that adverse effects to meditation are rare. Some manifestations in fact may not be at all harmful and may be signs of progress in meditation (e.g. changes in the perceived relative size of different parts of the body) but might be mistaken for “going crazy.” Other manifestations — such as some people who have contacted me to discuss a complete loss of emotional affect — are clearly very destructive and need to be investigated.
Britton makes the point that some approaches to meditation have ripped mindfulness from its traditional context, where it’s embedded in a framework of practice that includes intellectual understanding, ethical observance, devotion, and practices such as the brahmaviharas, and present it as a stand-alone practice. This may work for many people, but it may also lead to problems.