I would recommend extreme caution if you are asked to treat someone with a pacemaker. The effect of Reiki energy on such a device can be very unpredictable.
Extreme care should be taken when treating anyone who suffers from diabetes. Reiki can affect the levels of insulin required by the body. You will need to warn the recipient that if they wish to receive Reiki, they will need to monitor their insulin levels very closely.
Care should be taken during all stages of pregnancy with the use of essential oils on pressure points.
It is important to remember that while it is safe to give Reiki to babies and children, they do not need to receive Reiki for as long as an adult. Twenty to thirty minutes should be adequate for a full body treatment.
Do not attempt to give or receive Reiki after drinking alcohol as it may cause some unpleasant side effects.
Do not give Reiki to broken bones before the site of the break has been set in plaster. Otherwise there is a real chance that the bone may start to knit at an incorrect angle.
~from Healing Reiki