There is direct physical harm from THC. Not for everyone, not always, but it would be naive to assume there isn't.
Indirectly it's a bad idea to manage pain by just one way, be it pills or THC or whatever. The danger is that the conditions change or that the pain is telling you that you need additional medical attention.
The worst THC is likely to do is give somebody a bad trip, like Maureen Dowd's recent venture: http://www.forbes.com/sites/daviddisalvo/2014/06/09/why-eati...
Actual physiological harm shouldn't be a concern.