GlaxoSmithKline and Biota's Relenza, known generically as zanamivir, is an alternative to Tamiflu and has been shown to be active against Tamiflu-resistant virus.
But Relenza must be inhaled, making it problematic for patients with respiratory conditions such as asthma.
Glaxo hopes eventually to develop an injectable form of the medicine to overcome this limitation, but a spokeswoman in London says it is unlikely to be ready for at least two years.