What are the side effects and when should it be avoided?

As a general rule, reiki can’t do any harm because it is a form of non-invasive energy healing and will adjust itself to create the most appropriate effect for each individual person. However, although most people feel peaceful and relaxed after treatment, it can stir up emotions as part of the healing process. Some people also experience a healing reaction such as a headache or flulike symptoms, but any discomfort should be short-lived. (From Embodyforyou.com)