The calculation was made by the National Prescribing Centre in Liverpool, an advisory organisation, and sent to health service managers with a warning that the drug could "increase expectations dramatically".
Details published in Doctor, the GPs' newspaper, show that 4.8 million people could need the drug in an epidemic year, based on the numbers of people currently chosen for vaccination. At a cost of pounds 24 per dose, it would mean an NHS bill of pounds 115.2m.
Relenza, manufactured by Glaxo Wellcome, is the first of a new generation of flu drugs. It is available in the United States.
A spokesman for Glaxo Wellcome said it expected no more than 500,000 people to receive the drug in an average year, at a cost to the NHS of pounds 12m.
The drug is licensed for patients over 12 years old and recommended for those at risk from flu, such as the elderly and those with asthma and heart disease. Each year influenza claims more than 3,000 lives in Britain, mainly the elderly: nearly 150 million working days are lost because of it.Reuse content