Eucommia bark

The shortest answer and in purely scientific/research terms, no ones really knows.  While Chinese Medicine has been used for a very long time, very few long-term studies on individual herbs or even formulas for that matter have been done.  Same holds true for even simple vitamins from a western perspective.

From a Chinese Medicine perspective viewpoints will vary but we are generally wary of doing things without a very defined purpose (in TCM terms) and for longer than necessary to get the defined change.  In general, it's still diet, exercise, lifestyle choices that will contribute the most - but what is "diet" and could/should that not include herbs - that's a good question.

Eucommia Bark, Du Zhong, is considering a yang energy tonic in Chinese Medicine terms - it falls into the "herbs that tonify yang" category.  So in pure TCM terms if you are truly yang deficient and have other issues directly related to the functions of the herb then it is possibly a good idea for your to use it to help resolve those issues.  This is where it gets a little sticky, however, - and, again, viewpoints vary - but Chinese Medicine isn't generally fond of using individual herbs, focusing instead on properly balanced formulas that are directly tailored to the individual ("Choosing an appropriate herbal formula" gets into this).

So the purest answer to your question is that you would likely do better consulting with a Chinese Medicine practitioner and getting on a formula that is perfectly designed for you.  Where this matters is when you consider joint issues (again, very generally) can come from yang deficiency or yin deficiency among other diagnostic factors - the articles ("My Liver is What?" and "My Kidneys are What?" get into these diagnostic aspects).  If your underlying issues are more yin deficiency (more likely in a 30 yo woman), then too much yang tonics can actually make the underlying system worse, or at least challenge it.  And that line between yang deficiency and yin deficiency can be difficult to ascertain in generally healthy individuals.  So in that diagnostic sense, the wrong herb taken for longer periods particularly one that is completely wrong for your overall situation in Chinese Medicine terms can make you worse and, perhaps more importantly, can contribute to bigger issues down the road.