Reclast is one brand name for zoledronic acid. (It is also named Zometa, Zomera, or Aclasta). It stimulates bone growth and is used to treat osteoporosis. This drug is so slowly cleared from your body that it is given as a once a year injection. It is closely related to the more common osteoporosis treatment, alendronate (Fosamax).
Zoledronic acid, alendronate, or similar drugs in the category known as "bisphosphonate" drugs do not increase or decrease your risk for breast cancer. Side effects of bisphosphonate drugs include esophagus irritation (if the oral form is used). In rare cases, these drugs can lead to a painful jaw condition called "osteonecrosis." These drugs can't be used unless you have normal or near-normal kidney function.
If you want to treat thin bones, and have a history of breast cancer, one treatment you want to avoid is the hormone estrogen. Estrogen promotes breast cancer growth, so it is an especially bad choice for a person with a history of breast cancer.
Some drugs are "modified" estrogens. They can help bone strength, and work to prevent breast cancer. Examples are raloxifene (Evista) and tamoxifen (Nolvadex and other brands). These drugs have potentially harmful side effects. For example, they increase the risk for a blood clot in the leg.
Discuss all these treatment options with the doctors who are treating your breast cancer. They can help you to think through the benefits and risks.