Can I use a road bike stem on my mountain bike? I've got a XC mtb which 1. I'm trying to cut the weight off , 2. I want a bit extra reach. I've got a longer road bike stem made by 'Deda' (I think Planet X)? and I was wondering if it will be strong enough for XC mountain biking use?
For XC and trail use it'll be fine.

It's really only downhill bikes that require especially strong stems, I think (and in that case you'd likely be using a shorter stem anyway for handling reasons).