Hypnosis might not be appropriate for a person who has psychotic symptoms, such as hallucinations and delusions, or for someone who is using drugs or alcohol. It should be used for pain control only after a doctor has evaluated the person for any physical disorder that might require medical or surgical treatment. Hypnosis also may be a less effective form of therapy than other more traditional treatments, such as medication, for psychiatric disorders.

Ernest Hilgard, who developed the "neodissociation" theory of hypnotism, hypothesized that hypnosis causes the subjects to divide their consciousness voluntarily. One part responds to the hypnotist while the other retains awareness of reality. Hilgard made subjects take an ice water bath. None mentioned the water being cold or feeling pain. Hilgard then asked the subjects to lift their index finger if they felt pain and 70% of the subjects lifted their index finger. This showed that, even though the subjects were listening to the suggestive hypnotist, they still sensed the water's temperature.[180]
The real origin and essence of the hypnotic condition, is the induction of a habit of abstraction or mental concentration, in which, as in reverie or spontaneous abstraction, the powers of the mind are so much engrossed with a single idea or train of thought, as, for the nonce, to render the individual unconscious of, or indifferently conscious to, all other ideas, impressions, or trains of thought. The hypnotic sleep, therefore, is the very antithesis or opposite mental and physical condition to that which precedes and accompanies common sleep

The first neuropsychological theory of hypnotic suggestion was introduced early by James Braid who adopted his friend and colleague William Carpenter's theory of the ideo-motor reflex response to account for the phenomenon of hypnotism. Carpenter had observed from close examination of everyday experience that, under certain circumstances, the mere idea of a muscular movement could be sufficient to produce a reflexive, or automatic, contraction or movement of the muscles involved, albeit in a very small degree. Braid extended Carpenter's theory to encompass the observation that a wide variety of bodily responses besides muscular movement can be thus affected, for example, the idea of sucking a lemon can automatically stimulate salivation, a secretory response. Braid, therefore, adopted the term "ideo-dynamic", meaning "by the power of an idea", to explain a broad range of "psycho-physiological" (mind–body) phenomena. Braid coined the term "mono-ideodynamic" to refer to the theory that hypnotism operates by concentrating attention on a single idea in order to amplify the ideo-dynamic reflex response. Variations of the basic ideo-motor, or ideo-dynamic, theory of suggestion have continued to exercise considerable influence over subsequent theories of hypnosis, including those of Clark L. Hull, Hans Eysenck, and Ernest Rossi.[40] It should be noted that in Victorian psychology the word "idea" encompasses any mental representation, including mental imagery, memories, etc.
Remember this is real and not a trick. If you choose to use these powerful tools of suggestion you can help people in pain. If they are depressed or stressed, you can make them feel more at ease. You can help someone relax before a public speaking event. It can be therapeutic and help in defeating addictions and bad habits when used with other tools. This is how hypnosis meant to be used. Hypnosis can be used in social situations as long as the person doing the hypnotic induction has the proper training. Don't trust any stranger you meet! Remember: Use these techniques responsibly and only after you have been trained in hypnosis.

In the brain, this state looks stranger still. A landmark study in the prestigious journal Science in the late 1990s, led by Pierre Rainville of the University of Montreal, described a study in which hypnotized people briefly placed their left hand in either painfully hot water, heated to 116 degrees Fahrenheit, or room-temperature water. Some of them had been told that they would be experiencing pain, but that they wouldn’t be very bothered by it — if, on a scale of one to ten, the hurt would normally register at an eight, they’d feel it as if it were a four. As all the participants placed their hands in the 116-degree water, their brains were scanned. The results were clear: Those who had been told that the pain would be less intense showed less activity in their brains — specifically, in the anterior cingulate cortex, which is associated with pain processing.
During your first session, you will likely begin by telling the therapist about your goals and issues. You will then work together to come up with a treatment plan. Once you enter a state of hypnosis, your body will feel calm and relaxed, even as you enter a state of increased awareness, similar to the way you might feel when meditating. Your therapist will speak to you in a calm and gently assertive voice, and place the suggestions you agreed to in your treatment plan into your subconscious mind.

Finally, the neural underpinnings of PHA will be even clearer when we incorporate its most important aspect in imaging studies—the dissociation between implicit and explicit memory. In PHA (and in functional amnesia) the person is unable to explicitly recall certain information, yet we see evidence of this material on implicit measures. For instance, a participant given PHA may fail to recall the word “doctor,” learned earlier, but will have no trouble completing the word fragment “d _ _ t _ r”. Mendelsohn et al. did not assess implicit memory. Rather, they tested recognition, which in a sense confounds explicit and implicit memory. We’d like to compare brain scans of a PHA group trying to explicitly recall the movie (they should show reduced activation, as above) with brain scans of the same group completing an implicit memory measure of the movie (they should show normal activation). This would be tricky to do—implicit measures of complex material such as movies and autobiographical memories are hard to find or construct. But it would contribute to a more complete neural picture of the processes involved in these fascinating forms of forgetting. 

Mendelsohn et al.’s study is important because it demonstrates that hypnotic suggestions influence brain activity, not just behavior and experience. Hypnotic effects are real! This fact has been demonstrated clearly in earlier work, for instance, by psychologist David Oakley (University College London) and colleagues, who compared brain activation of genuinely hypnotized people given suggestions for leg paralysis with brain activation of people simply asked to fake hypnosis and paralysis.
Since hypnotherapy is an adjunct form of therapy, used along with other forms of psychological or medical treatment, there are many applications. Hypnotherapy can be used to treat anxiety, phobias, substance abuse including tobacco, sexual dysfunction, undesirable spontaneous behaviors, and bad habits. It can be used to help improve sleep, learning disorders, communication, and relationship issues. Hypnotherapy can aid in pain management and help resolve medical conditions such as digestive disorders, skin issues, and gastrointestinal side effects of pregnancy and chemotherapy. It can also be used by dentists to help patients control their fears or to treat teeth grinding and other oral conditions.