From looking at the advice of various manufacturers and professional bodies, below is an example of some conditions and situations where a heating pad is said to not be suitable.
However this isn’t a comprehensive list (nor does it constitute medical advice) so it is important to always check with a medical professional before beginning treatment.
Do not use on any parts of the body that are swollen, injured or inflamed
Do not use on a child under the age of 8
Do not use if you are not sensitive to heat
Do not use if you may not be able to react to over-heating (e.g. diabetics, persons with skin changes related to illness or persons with scarred skin in the area where the pad is to be applied)
Do not use when under the influence of drugs or alcohol