Campaigners are appealing against a decision they claim is restricting doctors from prescribing osteoporosis drugs on the NHS.
Osteoporosis leaves bones vulnerable to fractures
Currently only one drug - alendronate - is approved by the National Institute for Health and Clinical Excellence.
But the drug can cause a severe reaction in a quarter of patients.
The National Osteoporosis Society (NOS) says hundreds of thousands of people are missing out on potentially life-saving treatment as a result.
NOS says the decision by NICE to recommend the cheapest drug is a false economy and leaves many patients at increased risk of painful and life-threatening fractures.
Professor Ignac Fogelman of the NOS said: "We are challenging the financial model that was used to look at the cost effectiveness of the various treatments for osteoporosis."
GP Rosemary Leonard said: "Alendronate is the one osteoporosis drug which is now off patent so it is a lot cheaper than the others which is why there is this push to prescribe it.
"But unfortunately one in four people who are on it can get bad reactions to it, particularly inflammation of the oesophagus.
"There is a choice available but NICE guidelines say that primary care trusts only have an obligation to provide alendronate."
In some primary care trusts, this means alendronate is the only drug choice offered, she said.
"We are heading for a postcode lottery."
Professor Tim Spector, a consultant rheumatologist at St Thomas' Hospital, London said: "It is vital for clinicians and patients to have alternative treatments available so we can maximise patient choice, reduce avoidable drug side effects and reduce the risk of osteoporotic fractures."
NICE has not yet released its final guidance.
It said in a statement: "It is disappointing that the appeal will delay final guidance on use of drugs for osteoporosis and delay publication of our clinical guideline which will then set out the best use of drugs and non-drug treatments."
A spokeswoman said NICE would report back in due course once the appeal had been heard.
Osteoporosis literally means "porous bones" and makes it more likely for them to fracture as they lose their density.
Over 1m women in the UK have been diagnosed with the disease, although experts say many more probably suffer from the condition.