A shake-up of labelling for the bisphosphonate class of osteoporosis drugs looks on the cards, following a US Food and Drug Administration advisory committee vote on Friday.

The headline news from the panel meeting was a 17 to 6 vote in favour of rewording labels to spell out how long patients should receive them, potentially setting an upper limit for duration of use.

Earlier this year, bisphosphonate labels were modified to include a recommendation that "continued therapy is re-evaluated on a periodic basis" to reflect the potential adverse reactions that can occur with protracted exposure to the drugs.

An FDA report compiled ahead of the advisory panel meeting indicates that over time treatment with bisphosphonates may decline in efficacy, potentially altering the benefit-to-risk ratio for side effects linked to their use, such as atypical fractures of the thigh, osteonecrosis of the jaw, and oesophageal cancer.

While bisphosphonates have not been definitively proved as causing either atypical fractures or jaw osteonecrosis, epidemiological evidence suggests that risks may increase over time. Meanwhile, epidemiological studies on oesophageal cancer associated with bisphosphonates have yielded "conflicting results," according to the agency.

The panel voted to update the labelling but could not agree on maximum duration of use, noting that there was little data on efficacy and safety beyond the three- to five-year timeframe.

Generic alendronate currently dominates the market in terms of the volume of prescribed drugs, following the loss of patent protection for Merck & Co's Fosamax brand in 2008. Other bisphosphonates on the market include Roche's Boniva (ibandronate), Procter & Gamble and Warner Chilcott's Actonel/Atelvia (risedronate) and Novartis' Reclast (zoledronic acid).

Labelling changes could have an impact on the $7.6 billion market for bisphosphonates, given that a sizeable proportion is taken up by maintenance dosing and for osteoporosis prevention and patients often receive treatment with the drugs over a protracted period.

That said, the FDA notes that relatively few (2%) of patients actually receive treatment for five years or longer.