hey there
as the others have said it is not recommended, i think bc of getting into your blood stream.
interestingly some of the heat packs that you stick on and that last 12 hrs or whatever are also not recommended, as they contain ingedients that can cause early labour... so be careful whcih ones you get (i got advice from royal women's hosp in melb or the pharmacy list) - i think elastoplast brand are ok but some of the others not... to be safe o try to use a wheat pack most of the time and just the stick on ones when going out if back is v v bad (which it has been for a while now).
Hope that helps and hope you get some relief soon!