Two years after completing treatment for breast cancer, Mercy Moore knew there was a problem — she just couldn’t fix it. It was a persistent pain in her lower back.

“I went to urgent care and was told it was sciatic nerve,” Moore said. “It went away after I went to the chiropractor several times.”

Months later, the pain returned. When other treatments failed, an orthopedic doctor ordered tests that revealed the breast cancer had spread. It was in an area near the left side of the sacrum, the triangular bone at the base of her spine. The mother of two boys was in tears.

In a phantasmagoric scene, a few days before I spoke to Moore, my oncologist at the University of Miami’s Sylvester Comprehensive Cancer Center was prescribing me a new drug.

“There is yet one more medication I would like you on. It couldn’t be easier,” said Dr. Marc E. Lippman. “It’s called Prolia and it is a little subcutaneous shot, like the flu shot, every six months.”

At about $585 per shot, those investing in the drug appear to be teeming with good fortune. Amgen reported worldwide sales at $33 million for 2010. The Wall Street Journal quoted a reputable analyst projecting annual worldwide sales of the drug at $5 billion by 2015.

Amgen, an international biotechnology company, is both the developer and manufacturer of the drug, which is also known as Denosumab. It is being marketed as the first of its kind, because it is a laboratory-produced molecule known as a human monoclonal antibody. These cloned human molecules are made to target and destroy specific cells.

“One of the main reasons it was developed is to toughen up bones for osteoporosis,” Lippman said. “If you are postmenopausal then you are at risk for osteoporosis so it’s a good idea.”

I’m currently taking a drug called Tamoxifen that makes it difficult to tell if I am postmenopausal. The idea was alarming. Osteoporosis is the bone-thinning disease that made the last years of my grandmother’s life painful. The illness can cause squashed backbones and broken hips. According to the National Osteoporosis Foundation, about 34 million women in the U.S. are at risk.

The U.S. Food and Drug Administration approved Prolia for this purpose in 2010. It was meant mostly for high-risk elderly patients. So how did I end up receiving this drug? I think my heart skipped a beat when Lippman said that women who get it every six months have a vastly lower incidence of cancer recurrences in bone.

“In the same exact sense that fluoride toughens up your teeth and makes them resistant to bacterial attack, this medicine changes a little bit the structure of your bones, and makes it very resistant to breast cancer cells.”

I later found out that neither the FDA, nor Amgen has released public information supporting this claim. In February, the Food and Drug Administration’s oncologic drugs advisory committee voted that the drug did not have a favorable risk-benefit profile as a treatment to reduce the risk of bone metastases in men at high risk.

“It’s quite effective, and several of the most recent trials even suggest that it improves survival, so there is almost no downside to this medicine,” said Lippman. “You shouldn’t have side effects at all.”

So why weren’t all cancer survivors lined up to get the shot? My ignorance was making everything sound like hyperbole. I later read the maker of the drug reported a dossier of possible adverse reactions that included skin infections and pancreatitis. The European Medicine agency also reported infections of the urinary and respiratory tract, and cataracts. Lippman did warn me about one side effect. The drug can occasionally cause trouble with the jawbone.