disregard the review which say these microwavable heat pads don't work, they do i have used them for years and if heated correctly depending on your microwave they should last for years. if you are using a high powered microwave you should only heat the pads for one minute each side,if still not warm enough increase by 15 seconds at a time, you have to check you are not overheating other wise the pad will burst.Will stay warm for about two hours in the cover.
these microwave heat pads are great when you feel cold just hug one of these , use them to heat your bed, great for joint aches.
also ideal for keeping takeaway food warm, use the heat pad without the cover in between containers to keep food warm when travelling
There is a website that sells spare Hotties heat pads in case of acccidental overheating.