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.
Occasionally people experience a temporary aggravation or intensification of symptoms during or after a Reiki session. This may be as simple as a momentary feeling of discomfort at the site of an old injury or surgical scar. Such an experience resolves quickly and may be part of the body's process of healing.
A temporary aggravation of symptoms sometimes occurs when people suffering from chronic conditions elect to receive multiple (and perhaps longer than usual) Reiki sessions in quick succession.
In this scenario, the person initially feels better-a general improvement in wellbeing  and/or relief from specific symptoms-followed by a transient period in which the recipient either feels very tired and/or the symptoms return. Traditionally, this is seen as a positive response that indicates that the body's healing mechanisms have been stimulated and the body is actively engaged in a healing process. A hallmark of this process is that the client is not anxious about the return of symptoms, but intuits that this is simply the body doing what it needs to do.
Continuing Reiki through this period brings comfort, reduces symptoms, and speeds the return of wellbeing. However, if there is any doubt, it is important to seek out whatever medical advice or health care is deemed appropriate. A physician or nurse practitioner, for example, may be able to offer guidelines, advising that the situation does not need medical treatment as long as it resolves within a certain period of time.