Quotations from different sources:

"There is a common misconception in the West that hatha-yoga, one of about ten forms of Yoga that supposedly leads to self-realization, is merely a neutral form of exercise, a soothing and effective alternative for those who abhor jogging and calisthenics ... [However], Hatha-yoga is 'one of the six recognized systems of orthodox Hinduism' and is at its roots religious and mystical. It is also one of the most difficult and potentially [spiritually] dangerous forms of Yoga.
"The term hatha is derived from the verb hath, which means 'to oppress.'... What the practice of hatha-yoga is designed to do is suppress the flow of psychic energies through these channels ["symbolic, or psychic passages on either side of the spinal column"], thereby forcing the 'serpent power' or the kundalini force to rise through the central psychic channel in the spine (the sushumna) and up through the chakras, the supposed psychic centers of human personality and power. Westerners mistakenly believe that one can practice hatha-yoga apart from the philosophical and religious beliefs that undergird it. This is an absolutely false belief. ... You cannot separate the exercises from the philosophy. ... 'The movements themselves become a form of meditation.' The continued practice of the exercises will, whether you ... intend it or not, eventually influence you toward an Eastern/mystical perspective. That is what it is meant to do! ... There is, by definition, no such thing as 'neutral' Yoga" Johanna Michaelsen Like Lambs to the Slaughter, pp. 93-95

“A woman of my acquaintance upset her hormonal balance doing
this yoga exercise, and it produced a malfunction in her adrenal glands. Doctors didn’t know how to reverse the effects... and she soon died...Swami Rama warns that advanced forms of patterned breathing, which is a common yoga exercise, can cause a person to harm himself irreparably.” . Richard Kieninger, The Spiritual Seekers’ Guidebook, Quinlan, TX: The Stelle Group, 1986, p. 71.

“To practice pranayama [breath control] without real guidance is very dangerous. I know of three persons who have died from it…”
Sri Chinmoy, Astrology, the Supernatural and the Beyond, Jamaica, NY: Agni Press, 1973, pp. 53-68,87-89; Sri Chinmoy, Conversations with the Master, Jamaica, NY: Agni 4

“Now we come to breathing exercises. Let me caution you: they can be very
dangerous. Unless properly done, there is a good chance of injuring the brain. And those who practice such breathing without proper supervision can suffer a disease which no known science or doctor can cure. It is impossible, even for a medical person, to diagnose such an illness.... [For example,] I had known a young boy of perhaps 16 or 17 years of age who had begun to practice hatha yoga.... He was acting very strangely. He would prostrate fully on the ground, rise to full height, then repeat
the performance—over and over again. The Swami said that he had lost his mind. ...Finally, however he became so unmanageable that he had to be confined.... As regards breathing exercises, I know that Sri Ramakrishna, Holy Mother, and all the disciples of Ramakrishna have warned us again not to practice them (yet Vivekenanda, Ramakrishna’s disciple, encouraged them! “
Swami Prabhavananda, Yoga and Mysticism, Hollywood, CA: Vedanta Press, 1972, pp. 18-19.

“Yoga is not a trifling jest if we consider that any misunderstanding in the practice of yoga can mean death and insanity,” and of kundalini yoga, he says that if the breath is “prematurely exhausted [withdrawn] there is immediate danger of death for the yogi.”
Hans Ulrich Rieker, The Yoga of Light: Hatha Yoga Pradipika, New York: Seabury
Press, 1971, pp. 9,134.

“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 discipline of Hatha yoga.”
Gopi Krishna, “The True Aim of Yoga,” Psychic, January-February, 1973, p. 13.

“Just as lions, elephants, and tigers are tamed, so the prana, should be kept under control. Otherwise it can kill the practitioner.”
The Hatha Yoga Pradipika (chapter 2, verse 15), Rieker, Yoga of Light , p. 79.

“As stated before nothing but dangerous, mediumistic psychisms or neurotic dissociations of personality can result from the practice of [yoga] meditation without the qualifications mentioned at the end of the last chapter.”
D. R. Butler, “Instant Cosmic Consciousness,” in John White, ed., Kundalini Evolution and Enlightenment, Garden City, NY: Anchor, 1979, p. 47.

“To practice it, as many do, out of curiosity...is a mistake which is punished with futility, neurosis, or worse [‘even insanity itself’].”
Sri Krishna Prem, The Yoga of the Bhagavat Gita, Baltimore, MD: Penguin, 1973, pp. XV, 46.

“considerable pain, physical disorder, and even disease….”
Avalon, Serpent Power, p. 12.

Ernest Wood warns of “the imminent risk of most serious bodily disorder, disease, and even madness.” He observes that many people have brought upon themselves incurable illnesses or insanity by neglecting Hatha Yoga prerequisites, and “by any mistake there arises cough, asthma, head, eye, and ear pains, and many other diseases.”
Ernest Wood, Seven Schools, p. 14.

“All features associated with European witches are claimed also by Indo-Tibetan yogis and magicians.”
some yogis: “... boast that they break all the religious taboos and social rules: that they practice human sacrifice, cannibalism, and all manner of orgies, including incestuous intercourse, and that they eat excrement, nauseating animals, and devour human corpses. In other words, they proudly claim all the crimes and horrible ceremonies cited ad nauseam in the Western European witch trials.”
Mircea Eliade;leading authority on shamanism and comparative religion. Editorial, Yoga Journal , May/June 1984, p. 71.

“People forget that Yama and Niyama [limbs one and two] form the foundation [of yoga practice], and unless it is firmly laid, they should not practice postures and breathing exercises. In India and Europe, I came across some three hundred people who suffered permanently from wrong practices, the doctors on examination found there was nothing organically wrong and consequently could not prescribe.”
Bhagwan Shree Patanjali, Aphorisms of Yoga, trans. Shree Purohit Swami, London: Faber and Faber, 1972, pp. 56-57.

A personal story:

…….”Carole... used yoga for medical and health reasons. We published her story in The Coming Darkness: Confronting Occult Deception. (1) We first met Carole as a result of exchanging information on the famous Indian guru and yogi Swami Rama. The following information is taken from material sent to us.
Carole was very sick and doctors were unable to find the cause of her illness. When she went to a physician-nutritionist recommended by a friend, she found some literature in his office about the Himalayan Institute, of which the doctor was a staff member. The institute was founded by Indian Swami Rama, one of the most scientifically studied of the gurus, beginning with famous biofeedback researcher and spiritist Dr. Elmer Green. Carole
decided to attend the institute, where she began lessons in hatha yoga. Eventually, she was initiated and received her mantra, or word of occult power, from Swami Rama. As he laid his hands upon her head, the typical transfer of “occult energy” began (termed shaktipat diksha). Carole was in heaven:

Currents of electrical energy began to permeate my head and went down into my body.... It was as if a spell had come over me, the bliss that I felt was as if I had been touched by God. The power that had come from his hand, and simply being in his presence, drew me to him irresistibly.

The night after receiving her mantra, Carole was visited by a spirit being who claimed to be the spirit of Swami Rama himself. Although no one had ever mentioned the spirit world in her church (they did not believe in such things), Carole felt that this was the means of directly communing with God. She experienced wonderful powerful forces and energies, while thoughts entered her mind with a magnetic-like force:

Electrical currents were pulsating around my body and then moved into my
hand, the currents were shaking my hand and strong, almost entrancing thoughts were impressed into my mind, “Meditate, meditate. I want to speak with you.” It was a miracle. I was communicating with the spirit world. I had found God. Sitting in the darkness of my living room I began to repeat my mantra. A presence seemed to fill the room. I began to see visions of being one with the universe and the magnetic thoughts were now leaving and I was hearing a voice, which identified itself as Swami
Rama, saying he was communicating with me through astral travel.
Within one week, after meditating many hours each day and still in constant
communication with this spirit, forces began to come upon me and gave me powers to do yoga postures; I was floating through them, the forces giving me added breath even… postures that before would be very painful to do.

However, after two weeks of daily yoga meditation, Carole became engulfed in a nightmare of utter dread and terror. Voices that once claimed they were angelic turned threatening, even demonic. She was brutally assaulted, both physically and spiritually by spirits. During meditation,
in the midst of being violently shaken, she could sense that the same energy received at initiation, energy which was now felt to be personal, was attempting to remove her life-essence from her physical body—in her words, “to literally pull the life from my shell of a body.” She sensed an overwhelming and implacable hatred directed toward her from his “energy,” as if “monstrosities of another world were trying to take my very soul from me, inflicting pain beyond endurance, ripping and tearing into the very depths of my being.”
The intermittent suffocation and torment seemed interminable; her fears increased as she realized there was no one to help her. Finally, the attack subsided. But it was merely the first of many.
It seems that nothing could stop the assaults. Her agonized pleas to the spirits were ignored; her husband was powerless. Her father wanted her to see a psychiatrist; others also doubted her sanity. In desperation, her mother contacted psychic friends from a local church of the Unity School of Christianity. They laid hands on Carole and commanded that
“the divinity within” deliver her, but to no avail.
Dr. C. Norman Shealy, M.D., Ph.D., entered the picture. He is a noted neurosurgeon, a former professor at Harvard University, past president of the American Holistic Medical Association, and the author of Occult Medicine Can Save Your Life. Dr. Shealy also works in conjunction with psychics and spiritists such as Caroline Myss. When Dr. Shealy was unable to help, he referred Carole to Dr. Robert Leichtman, M.D., a spiritist who is coauthor
of several dozen books received by revelation from the spirits.
Leichtman admitted that Carole’s situation was not uncommon among followers of Eastern gurus. He even told her some have died as a result of similar psychic attacks. But he, too, was unable to help. His instructions, such as visualizing herself in the white “Christ light” of protection, were useless. By this time, Carole was near the end.

I had to endure the torture, unable to free myself. To those around me I was insane. No one believed me and no one could free me. The hopelessness I felt was unbearable. No one believed me except the psychics... and they could do nothing.
I was defenseless against these never-ending attacks... hundreds of presences filling my room, which itself would be filled with thick, ice cold air, my body drenched with perspiration as my whole being fought against them.
After spending several weeks at my parents’ we decided perhaps I could try
returning home. But that night the spirits started to exert their full power.
First, against my skull. I felt as if they were trying to crack it open, like the air was being cut off to my brain. Incredible pressure was exerted upon my back and chest, pulling with a wrench-like grip. It felt like they were trying to pull my shoulder from its socket, pressing on my eyes trying to blind me, pushing on my throat trying to choke me. Filled with fear and exhaustion, on the brink of death I screamed to my husband, “I’m dying; I can’t take it anymore. Get me to the hospital.”
I was taken to the hospital where I laid like a scared dog cowering on a cart. I could hardly speak but at least the spirits were gone—temporarily.... The doctor on duty recommended a psychiatrist who saw me the next morning. He told me I was covering up some deep problems with this “talk of evil spirits.” “There is no such thing as the devil,” he said coldly.

Carole admitted herself to the hospital, but once more no one could help. The attacks finally subsided and she was released. Upon returning home, the attacks began again. More unimaginable torment. Although she was terrified of dying, death was now her desire. Wishing to take her life but too fearful of dying, she readmitted herself to the hospital. Once again, she was placed in locked ward. She felt that here she would die, alone and in torment But today, Carole is alive and well. Even her psychiatrist is amazed at the miraculous transformation. She is now in perfect health, both mentally and physically.
How did Carole get free? No one had been able to help her. Today, Carole attributes both her health and her life to a living Jesus Christ who delivered her from a desperate plight. Reflecting back on her predicament, she is awed that such terrible destruction could be purchased at the price of a simple, supposedly harmless form of yoga meditation.
Events like these reveal that there is more to yoga than meets the eye. Whether yoga can trigger some unknown psychospiritual, physiological response, or whether changes are produced spiritistically, or both, few can deny yoga is a powerful spiritual discipline that has been used for millennia to secure occult, pagan goals. As we proceed, we will better understand
the reasons for this.

Anmärkning:
1. Book by John Ankerberg, John Weldon, The Coming Darkness: Confronting Occult Deception, Eugene, OR: Harvest House Publishers, 1993.
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