"One often hears and reads about the dangers of Yoga, particularly of the ill-reputed Kundalini Yoga. The deliberately induced psychotic state, which in certain unstable individuals might easily lead to a real psychosis, is a danger that needs to be taken very seriously indeed. These things really are dangerous and ought not to be meddled with in our typically Western way. It is a meddling with Fate, which strikes at the very roots of human existence and can let loose a flood of sufferings of which no sane person ever dreamed. These sufferings correspond to the hellish torments of the chönyid state... "
C.G. Jung; The Tibetan Book Of The Dead
"The fact that Hatha Yoga practices can lead to insanity is widely known in India and, to some extent, even in the West. The term "mastana" in Persian and "avadhoot" in Sanskrit is applied to an initiate whose entry to higher dimensions of consciousness is attended with loss of worldly sense to an extent as to be oblivious of his behavior or, in other words, who while attaining to higher perception loses control over himself. Cases of this category can be met in India and, probably, in other places also. All this association of Kundalini and its products with possible derangement of mind, during the course of practice and even later, can definitely mean, even if we disbelieve other things, that the practices of Hatha Yoga or the arousal of the Serpent Power can have such a drastic effect on the body and the brain as to cause unhingement of reason in some practitioners. Apart from every other consideration, does not even this issue provide a strong incentive for an earnest scientist to start an investigation on this type of Yoga? If the practices or the alleged arousal of the power can, in some cases, lead to psychosis, looked at from the other angle, can it not be possible that the spontaneous arousal of the power, brought about by genetic factors in an unadjusted system, is a fertile cause for many forms of insanity and other mental or nervous disorders?"
(…) In Hatha Yoga the breathing exercises are more strenuous, attended by some abnormal positions of the chin, the diaphragm, the tongue, and other parts of the body to prevent expulsion or inhalation of air into the lungs in order to induce a state of suspended breathing. This can have drastic effects on the nervous system and the brain, and it is obvious that such a discipline can be very dangerous. Even in India, only those prepared to face death dare to undergo the extreme disciplines of Hatha Yoga."
Gopi Krishna; The Awakening of Kundalini
"The Hatha-Yogi thus seeks a body which shall be as strong as steel, healthy, free from suffering and therefore long-lived. Master of the body, he is master of both life and death. His lustrous form enjoys the vitality of youth. He lives as long as he has the will to live and enjoy in the world of forms. His death is the "death at will", when making the great and wonderfully expressive gesture of dissolution he grandly departs. But it may be said the Hatha-Yogis do get sick and die. In the first place, the full discipline is one of difficulty and risk, and can only be pursued under the guidance of a skilled Guru. As the Goraksa-Samhita says, unaided and unsuccessful practice may lead not only to disease, but death. He who seeks to conquer the Lord of Death incurs the risk of failure of a more speedy conquest by Him. All who attempt this Yoga do not, of course, succeed, or meet with the same measure of success. Those who fail, not only incur the infirmities of ordinary men, but others brought on by practices which have been ill pursued, or for which they are not fit."
Arthur Avalon (Sir John Woodroffe); “The Serpent Power – The Secrets of Tantric and Shaktic Yoga”
"Many books have been written upon Yoga practices - some of them, I fear, by people who have little practical acquaintance with the subject; and in a number of cases harm has resulted from ill-judged attempts to follow the directions given. I am told that there are Indian Yogis who give instruction in these arts ; but the Yogi usually teaches only those who are definitely his pupils and follow him everywhere. He therefore has his experimenters always under observation, and can at once check anyone who may be running into danger; whereas the man who learns his Yoga from a book has no such safeguard. I have myself received a large number of applications for help from persons who have seriously injured their brains, their nervous system, and their constitution generally, by plunging blindfold into this kind of psychism; and, sadly, often no effective help can be given. It is so easy to lose one's balance - so terribly difficult to regain it. That is why our beloved President has forbidden the sale of such books at any of the Theosophical shops under her direction.
The President at least has been most careful not to give any dangerous advice, and has explained to her pupils that they should at once stop all meditation if any dangerous symptoms appear - even such as a headache."
Annie Besant; The Inner Purpose of the Theosophical Society
Artificial Methods - dangers
"Too early an attainment of cosmic glimpses - that is to say before the individual's moral standard is fundamentally established - will thus be very unfortunate, for it will take place or will be attained in some kind of unnatural way and in a restricted form or as a substitute for genuine glimpses. Absolute and infallible cosmic sight can be obtained only by cleansing the mind of all lower tendencies such as ambition, pride, intolerance, dishonesty and desire for power. The resisting of these realities is the unavoidable price which has to be paid for fundamental cosmic sight. That resistance makes the human being "pure in heart", and so it is a natural and genuine way of training for the attainment of divine sight.
Any kind of training which is not synonymous with the resistance mentioned above must then be the same as a false training or an artificial method, and only in the most fortunate cases can bring those concerned into contact with some of the cosmic energies. But in that instance they have not been through sufficient moral development, and their spiritual bodies are still too feeble for this contact. Therefore these people will overstrain themselves mentally and become spiritual cripples, a condition that invariably causes fanaticism, which in the worst cases can be followed by mental weakness or insanity. And here it is that the individual meets the "guardians of the threshold" in the shape of his own unfinished nature. No one is able to "see God" except the "pure in heart". Therefore neither will we here in Livets Bog encourage artificial training in any form whatsoever but point exclusively to the way through morality. For with fundamental moral development the cosmic faculties will come about quite of themselves in a natural way in all their unfailing power and strength and without any danger or any risk to the individual."
Martinus; Livets Bog, vol. 1, sect. 208.