There are no chemicals or magnetic energy inputs from Neurofeedback so side-effects are rare.  You may experience short-lived negative effects after training, but these effects are mild and are usually gone the next day. The most common side effect is mental fatigue after a session because Neurofeedback is learning and learning uses a lot of energy. Other side effects such as dizziness are typically from medication but may be exacerbated by the improvements gained with neurofeedback. Usually, we can alter the training so that negative effects do not occur.