Things to consider
While Reiki is a safe practice, there are a few instances where extra caution may need to be taken. Although studies are not present (to my knowledge at the time of writing this), I was taught to not place my hands near or around where a pacemaker is located.
Additionally, if the client is taking medications (insulin is a good example), it is good to remind them to be mindful of listening to their bodies (ex, a diabetic taking insulin would want to monitor their blood sugars more closely).
It is also suggested that Reiki not be performed during surgery or over broken bones that have not set yet.
I am also very gentle and cautious with pregnant women.
Use your judgement and always be informed.