Like every procedure that tries to heal something, hypnosis has also many risks. And maybe because it's not a normal, medical procedure, and it's not even totally explained yet, it presents higher risks. We all know the risks of medicine, especially of surgery. Hypnosis has even higher risks because they are not all predictable. That's why, before starting a hypnosis treatment, we should all be aware of the risks.
Hypnosis means giving up totally to your self-control in the hands of the person who hypnotizes you. Everything takes place with your agreement, because without it it's impossible. The hypnotist needs your permission and allowance to enter your brain and thoughts, or else his trial will fail. The real risk begins after he manages to take over your brain, your thoughts and ideas. This is when you can't do anything anymore.
The first risk is represented by the hypnotist. Either not a professional hypnotist or even a person who doesn't want to help you but to harm you. In both cases, he can damage your memories, your emotions and feelings and he can create fears and anxiety inside your head. You can do things under his command that normally you wouldn't do and you can remember things you wanted to leave forgotten. The point is: if you're not sure on his professionalism, better don't allow anyone to hypnotize you! Not even as a joke!
The second risk, if first one was eliminated, is represented by your mind itself. The hypnotist can never know what he can wake up inside of you with his questions. You can remember dramas, sad experiences from your past, old paranoia or even psychosis. By waking up certain things inside your mind without knowing these things will harm you, the therapist can really twist your mind. When you wake up, you will probably wake up with all those memories or psychosis back and it will be pretty hard to face them all. That's why hypnosis therapists generally ask many questions so they can understand if your memories are positive or not. And if they are not, they simply delete them back or tell you to forget them again.
Another disputed risk is the one regarding the moment when you wake up. There is a theory that states people risk not to wake up anymore after a hypnosis session. It's not something that can happen to everybody, but it can happen to some persons according to their mind state.
So before trying hypnosis, make sure you are very well informed and you choose the right person to hypnotize you!