Reiki has not been found to have any adverse effects.
Because there is nothing about a Reiki session that can interfere with conventional medical care, Reiki has no known contraindications and can be used concurrently with any medical intervention.
The touch of Reiki is very light on or even off the body. The recipient is not required to ingest any substances, so there are no concerns about mixing substances (such as herbs) and prescription medications.
Reiki will not override the action of medical interventions, but rather support the patient while he or she goes through them, restoring balance in body, mind, and spirit to the degree possible. Patients who feel well even when fighting chronic illness are more likely to complete their medical treatment and be active partners in their health care.
Because the activation of Reiki pulsations in the practitioner's hands adjusts to the changing need of the recipient, and stops when appropriate, you can't get too much Reiki, no matter how long the practitioner's hands are in place.
All that said, remember that it is up to you to be responsible for your own medical care and seek appropriate help. Do not go to your Reiki practitioner for a diagnosis (unless he or she is also your healthcare provider) and do not avoid recommended medical tests or treatment. In an emergency, call 911. If you are Reiki-trained, you can safely offer Reiki by placing your hand anywhere on the victim (even if you're the victim) while awaiting the medics or en route to the hospital.