I personally would get a second opinion. Not because of any reason to distrust the doctor's suggestion, but because treating osteoporosis may require pre-thinking a long-term strategy over a number of years. And that means paying attention to which drugs, if taken, might make other good drugs not possible or work less well.

Fosamax, a biophosphate, doesn't preclude other possibly-better drugs later. But there are some newer drugs that work better if taken first. So treating osteoporosis is an evolving field as newer drugs have become available and a doctor with a specialty in treating it is most likely to know all of the options. Including when or if the somewhat-controversial estrogen therapy could help. (Being thin is associated with a lower general level of estrogen and possibly an earlier-than-average loss of the so-called estrogen-shield that protects bones.)

Or you could start on the Fosamax while looking for a second-opinion specialist. I just think, with those numbers, you might want an overall plan of action for comfort while you research other non-medical things you can do in addition to drugs. That way you'll know you're using all the tools currently available to address this and maybe reverse some of the numbers as well.