Short answer: Because it can be an early sign of an unusual thigh bone break. The Fosamax and Prolia labels describe atypical femur fractures that happen with little or no injury, and say patients often had a dull, aching thigh pain for weeks to months before a complete break. The labels say anyone on these medicines with thigh or groin pain should be checked for an incomplete fracture.
The Fosamax label (section 5.5) describes low-energy or low-trauma fractures of the femoral shaft, the long middle part of the thigh bone, reported during bisphosphonate treatment. They most often happen with minimal or no trauma, and can occur anywhere from just below the hip to just above the knee. Atypical breaks of other bones have also been reported, and they may affect both legs.
The label adds that these fractures can also happen in people with osteoporosis who have never taken a bisphosphonate, and that glucocorticoids such as prednisone may also cause them. The Prolia label (section 5.5) carries a similar warning for denosumab, and says atypical femur fractures in its trial program were seen as early as 2½ years into treatment.
The Fosamax label says patients reported prodromal pain, usually a dull, aching thigh pain, weeks to months before a complete fracture. It says anyone with a history of bisphosphonate use who has thigh or groin pain should be suspected of having an atypical fracture and evaluated to rule out an incomplete femur fracture, and that bone pain elsewhere should be considered too.
The Prolia label says patients should be advised to report new or unusual thigh, hip or groin pain. The Fosamax Medication Guide puts it in plain words: a break in the thigh bone can feel like a new pain in your hip, groin or thigh, and you should call your doctor right away.
Both labels say that a person with an atypical femur fracture should also be checked for signs in the other leg. The Prolia label says pausing treatment should be considered case by case, after weighing benefit and risk; the Fosamax label has similar wording for bisphosphonates. Those decisions belong to the prescriber, especially for Prolia, where the label warns about stopping without a plan; see what happens if you stop Prolia.
The ASBMR task force on long-term bisphosphonate use found that the risk of atypical femur fracture clearly rises with how long the medicine is taken, which is one reason labels ask for treatment to be reviewed over time; see how long you can take a bisphosphonate.
The labels' point is to get new thigh, hip or groin pain checked. Useful notes: when the pain started, which side, whether it is worse with standing or walking, any steroid use, and how long you have been on the bone medicine. Dose Track keeps the start date and each dose, with a notes field for symptoms.
Read more on Dose Track: Fosamax side effects · Prolia side effects · Actonel side effects · Joint pain