It has already been explained in other articles that life force energy is already present in human beings since birth. Its imbalance causes disease, which is re-balanced through Reiki. It is a simple system that does not use any equipment or physical medicine. During a Reiki treatment, the energy only passes through the practitioner but it is not his/her own energy. He/She acts only as a channel but the question that can Reiki cause any harm is still there.
Physical medicines contain various chemicals that may at times cause harm but since nothing physical is administered in Reiki treatment, question of harm through it does not arise. One can never be harmed by it even if it is used several times a day. There is no over-dosing or over-saturation in Reiki because it does not work like a physical medicine. Reiki energy with its supreme intelligence goes only to the benefit of the recipient and makes no mistake. It is pure love and fully positive.
If the practitioner makes a deliberate mistake, even then Reiki will not cause harm because in that case the energy will not flow. Only as much Reiki flows as is naturally required by the recipient. It only works in best interest of the recipient.
So, the answer in one word is ‘no’. Reiki is safe to use and has absolutely no side effects; it cannot cause harm. You may use it as much as you want to without worrying about any harm or side effects.