The method used to assess which drugs should be recommended for widespread use on the NHS is "flawed", researchers have said.
The formula used by the National Institute for Health and Clinical Excellence (Nice) should be scrapped, according to the authors of a European Commission-funded study.
The method, called quality-adjusted life years (QALY), fails to reflect personal differences about medical outcomes, they said.
The QALY method is used to assess the value of new drugs for patients in England and Wales. The method involves looking at the cost of using a drug for a year and weighs it against how much someone's life can be extended and improved.
The authors of the report, who presented their findings at a conference in Brussels, surveyed 1,300 people across the UK, Belgium, France and Italy about the way they rate different medical outcomes.
The responses varied "hugely", the European Consortium in Healthcare Outcomes and Cost-Benefit research (ECHOUTCOME) team said, adding: "The way people rate different medical outcomes is personal, and cannot be summed up by a neat formula."
Some of the foundations of the formula were scrutinised. For instance, the assumption that someone who prefers 10 years of healthy life to five years, will also prefer 10 years in a wheelchair to five years, did not always reflect the views of those surveyed.
Project leader Ariel Beresniak said: "The research provides robust scientific evidence that QALYs produce hugely inconsistent, wrong results, on which important decisions are being made. Agencies such as Nice should abandon QALY in favour of other approaches. European Health Technology Assessment agencies currently looking to adopt the Nice model must seriously reconsider."
Gerard Duru, emeritus research director in mathematics at the French National Centre of Scientific Research, said: "The underlying assumptions of the QALY outcome are very theoretical and are not verified in a real population. The QALY indicator is not a valid scientific scale. It is impossible to know what we are measuring, and therefore impossible to base a formula upon it."
However a Nice spokeswoman said the study was "rather limited", adding: "When we want to find out whether a new treatment provides more for patients than current practice, and whether any improvement in quality or length of life justifies the price the NHS is asked to pay for it, we need to use a measure that can be applied fairly across all diseases and conditions. The QALY is the best measure anyone has yet devised to enable us to do this."