What happens if you use an expired heat pack?

You know those heat pack/ body warmers/ whatever they're called? The ones where you open them and they heat up, for sports injuries or the cold or something like that? Well, I have one that expired in may of this year... yeah, just wondering if anyone knows what happens when you use it:P Does it just not work as well/at all, or does it become dangerous in any way?

thanks! ^.^ :)