Like everything else, there are 2 sides to a coin. There just isn’t any perfection in anything. So, hypnosis is the same.
One side effect of hypnosis could be regression. Regression is used in hypnosis to bring a person back in timeline to maybe the childhood time. Some people have unpleasant memory in the past and has too much emotional garbage attached to this particular memory.
If the person undergoes hypnosis and is regressed to the time of the bad experience, it may unleash inside her and brought back to the present time. That person may then suffer from distress simply because her bad memory was “digged up” again after so many years of suppression.
This side effect could be overcome if you have an experienced hypnotherapist doing the hypnosis.