Actually, yeah, the same idea hit me later, that a vitamin, or some sort of drug, could be used to help strengthen bones. As far as going back to earth, I don't think that would be a problem. Having dense bones wouldn't be an issue on Earth, I don't think.
Actually, (from prior experience with race horses where bone mass is an issue), supplements would not be enough to save bone mass and would not build it.
Bones go through a continual building and break down process. These processes are different for younger animals (humans). In formative years, bones are actually shaped and the shape adds to or takes away from their strength for life. Then as animals age, nutrition AND PHYSICAL EXERCISE affect how dense and how brittle they are. Sorry, but I forget the term for this process, but bones need to be stressed to stay strong. When they are stressed, they lay down more bone cells that strengthen them. When they aren't stressed, as the cells die and are not replaced.
So, if you don't want to have full gravity, you need to provide a way to stress the bones on a very routine basis.
As for nutrition calcium is probably among the most important. Low body calcium (needed for heart function, etc) will cause the body to rob it from the bones. This balance of bone/blood levels makes me think that a medication that could overcome the lack of gravity (vs, just aid the process here on earth) would risk killing a person by causing calcium to be 'sucked' out of the blood.
If you google around, you might find these new body armor type things that have been invented in Japan. It looks like a full body exoskeleton that has been invented to give additional strength to workers here on earth. BUT, it wouldn't take much to tweak that to provide appropriate strength 'training' on an 8 hour work shift.