The NHS is spending £18m a year on oxygen therapy for which there is no evidence of benefits, doctors are warning.
They called for further scrutiny of the potential benefits of the treatment after finding most COPD patients who claimed it helped were unable to tell the difference between oxygen and air.
A crossover trial assessed short-burst oxygen therapy in 22 patients randomised to activity supported by oxygen or air, then repeated 15 minutes later with the alternative cylinder. Only five patients were able to distinguish the two cylinders.
Study leader Dr Simon Quantrill, consultant in respiratory medicine at Whipps Cross University Hospital in Leytonstone, east London, said: 'Patients say it benefits them when often it doesn't. We need more research to determine if there is a subset of patients in whom it works, and when it works.'
He added: 'People don't think of oxygen as a drug. If it was a tablet it would be trialled.'
Dr Quantrill stressed his study assessed activities of daily living and not, for example, benefit during exacerbations.
The research was published early online by Thorax.