PARIS (AFP) - Doctors believe they have found a way to double supplies of the precious anti-flu drug Tamiflu thanks to a tactic pioneered in World War II. The idea is to use Tamiflu alongside a second drug that halves the amount of Tamiflu that is excreted from the body in urine. As a result, a single dose of Tamiflu could be used to treat two people. The helper drug, probenecid, is commonly used to stop the active ingredients of antibiotics from being filtered out of the blood by the kidneys, the British science journal Nature reported Wednesday. The technique was invented during World War II to "stretch" supplies of penicillin. It is still used today, notably to treat gonorrhoea and syphilis, where quick, high doses of antibiotics are needed to treat infected people. The idea of using probenecid to stretch supplies of Tamiflu was spotted last week by an emergency-medicine specialist, Joe Howton, medical director of the Adventist Medical Center in Portland, Oregon. While preparing for a talk on bird flu, Howton noted that in 2002 Roche itself published a study which said probenecid doubled the length of time Tamiflu was in the blood, doubled the blood concentration of Tamiflu's active ingredient and multiplied 2.5-fold the patient's total exposure to the drug. "It dawned on me that the data potentially represented a tremendous therapeutic effect," he told Nature. Studies are being proposed that will investigate the safety of using probenecid with Tamiflu. But probenecid is already widely used as a helper drug and has a long history of safety, which means that regulatory approval could be fast-tracked, Nature said. Demand for Tamiflu has rocketed as the H5N1 bird flu scare has progressed. The Swiss manufacturer, Roche, cannot cope with the backlog of orders and is considering awarding licences to other firms to make the drug. The firm has quadrupled production over the past two years but the current supply is thought to cover just two percent of the world's population, and most beneficiaries are overwhelmingly in rich countries. By comparison, the World Health Organisation (WHO) recommends that countries should stockpile enough antivirals for at least a quarter of their population. Tamiflu -- lab name oseltamivir -- is not a vaccine. It is used against conventional strains of flu; it has not been fully tested against bird flu, which is transmitted from poultry to humans; and it is unclear how or whether it would work if bird flu became more contagious. It is designed to block reproduction of the influenza virus after infection. If taken early enough, the drug can avert the worst effects of flu and shorten the duration of sickness.