Not recommended. If you're talking about sand, it is OK (don't forget to lower tire pressure... ) but forget about rocks, etc.
It isn't safe for the tire and not healthy for the rim, no matter what (good) experience other have made. You can't see a tire damage with your plain eyes and rim damages are sometimes unvisible too (especially when the rims gets damaged from the inside).
Get a second set of 18'' rims with offroad tires and you're set.