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Mayo Clinic - There's a very informative article on this site so please do visit. Here is a brief excerpt:

Myth: When you're under hypnosis, you surrender your free will.
Reality: Hypnosis is a heightened state of concentration and focused attention. When you're under hypnosis, you don't lose your personality, your free will or your personal strength.

Myth: When you're under hypnosis, the hypnotherapist controls you.
Reality: You do hypnosis voluntarily for yourself. A hypnotherapist only serves as a knowledgeable guide or facilitator.

Myth: Under hypnosis, you lose consciousness and have amnesia.
Reality: A small number of people who go into a very deep hypnotic state experience spontaneous amnesia. However, most people remember everything that occurred under hypnosis.

Myth: You can be put under hypnosis without your consent.
Reality: Successful hypnosis depends on your willingness to experience it. Even with voluntary participation, not everyone can be led into a hypnotic state.