Walsh R, Roche L. (1979). Precipitation of acute psychotic episodes by intensive meditation in individuals with a history of schizophrenia. American Psychiatry Association. August, 136(8): 1085-6.
Walsh (1979) reported a number of disturbing experiences during meditation, such as anxiety, tension, and anger. Walsh and Rauche (1979) stated that meditation may precipitate a psychotic episode in individuals with a history of schizophrenia. Kornfield (1979 and 1983) reported that body pain is a frequent occurrence during meditation, and that meditators develop new ways to relate to their pain as a result of meditation. Hassett (1978) reported that meditation can be harmful. Carrington (1977) observed that extensive meditation may induce symptoms that range in severity from insomnia to psychotic manifestations with hallucinatory behavior. Lazarus (1976) reported that psychiatric problems such as severe depression and schizophrenic breakdown may be precipitated by TM. French et al. (1975) reported that anxiety, tension, anger, and other disturbing experiences sometimes occur during TM. Carrington and Ephron (1975c) reported a number of complaints from TM meditators who felt themselves overwhelmed by negative and unpleasant thoughts during meditation. Glueck and Stroebel (1975) reported that two experimental subjects made independent suicide attempts in the first two days after beginning the TM program. Kannellakos and Lukas (1974) reported complaints from TM meditators. Otis (1974) reported that five patients suffered a reoccurrence of serious psychosomatic symptoms after commencing meditation. Maupin (1969) stated that the deepest objection to meditation has been its tendency to produce withdrawn, serene people who are not accessible to what is actually going on in their lives. He said that with meditation it is easy to overvalue the internal at the expense of the external.
These and other negative meditation outcomes are described in traditional sources. The path is "sharp like a razor's edge" says the Katha Upanishad.  St. John of the Cross wrote an entire book about the dark night of the soul.  Several hundred pages of Sri Aurobindo's collected works deal with the problems and dangers of his integral yoga.  A large part of Aldous Huxley's The Perennial Philosophy consists of admonitions from various spiritual masters about the difficulties encountered in contemplative practice,  and William James explores the negative side of religious life in The Varieties of Religious Experience.  These and other sources provide a wide array of warnings and directions for those entering a path of meditation. Though the rewards of contemplative practice can be great, they do not come easily.