Of 104 PCTs that replied to a Freedom of Information request, 32% said they had issued guidance advising GPs to limit prescriptions to 28 days' supply at a time. A further 11% said they promoted such practice.

• Click here to see PCTs with 28-day prescription limits

Such policies aim to reduce drug wastage and save costs, but just five PCTs had looked at the impact of their policy on prescribing costs, inconvenience to patients or workload costs for GPs and pharmacists.

GPC prescribing subcommittee chairman Dr Bill Beeby described such policies as ‘virtually evidence-free’.

Studies show that increases in GP and pharmacy workload outweigh any savings from reduced medicines waste, he said. ‘If you do the sums, it doesn’t add up.'

Dr Beeby added that increasing the volume of prescriptions being processed increased the risk of errors occurring at each stage of the prescription chain.

The British Thyroid Foundation calculates that thyroid treatment with levothyroxine costs 42% more for 28-day prescription periods than 82-day periods.

Dr Brian Karet, Diabetes UK’s medical advisor for primary care and RCGP clinical lead for diabetes, said a 28-day limit was ‘inconvenient for patients and generates extra work for practices’.

‘As long as you have compliance issues sorted out, I can’t see the benefit to anybody,’ he said.

The Patients Association has said such policies are inconvenient for patients and have ‘unintended consequences that are detrimental to patients’.

The DoH said there was no government directive to specify prescription length.

‘We recognise that prescriptions for longer periods may be more appropriate and more convenient for some patients and we would not expect any PCT guidance on this to be mandatory,’ a spokesman told GP.

Buckinghamshire GP Dr Andrew Webber has refused to bow to his PCT’s 28-day prescribing rule.

‘Checking and signing scripts is a very significant daily task for GPs,’ he told GP. ‘Rushing the job risks errors. Much of the work cannot be delegated to non-medical staff. 28-day prescribing doubles this workload - forever. I won’t do it.’

Professor John Wass, professor of endocrinology at Oxford University, said PCTs are ‘chasing a mythical pot of gold at the end of the rainbow with this kind of thinking’.

Professor Wass is also chairman of the Addison's Disease Clinical Advisory Panel. He said: 'The risks to patient safety from restricted access to essential medication are particularly severe for steroid-dependent Addison's patients, who have been know to die less than five days after running out of their steroid medication.'

One PCT that has examined the impact of its policy is NHS Hertfordshire. It introduced its policy following local audits with GPs and community pharmacists that demonstrated 28-day prescribing reduced the amount of medicines returned, unused, to pharmacies for disposal. The PCT has outlines the reasons for its initiative in a leaflet written for patients.