Over the past two days, drugstores islandwide have faced an army of customers brandishing doctors' prescriptions for the antiviral drug, which comes in a box of 10 capsules, each pill costing around $5.

Others who do not have prescriptions are asking how to buy it.

At least 20 Watsons and Guardian pharmacies contacted said their small stores had run out and they had ordered more from the manufacturer.

A Watsons pharmacist told The Straits Times she had sold all her 14 boxes. She said: 'We have ordered more from our agent but the agent is also overwhelmed by orders. New boxes may come in next week.'

Guardian Health & Beauty confirmed that stocks at its pharmacies had been depleted by the sudden surge in demand, which has increased by 10 times over the last few days.

Private clinics, faced with a rise in requests, especially from Singaporeans travelling to affected regions, have been writing more prescriptions.

Raffles Medical Group, which runs 66 GP and specialist clinics islandwide, said 'a handful' of people had been prescribed Tamiflu over the past couple of days.

One of its senior family physicians, Dr Rupert See, said the drug is being prescribed either to 'serious cases of flu' or to people travelling to countries such as the United States, which have confirmed cases of swine flu. He said the clinic was not dispensing the drugs to patients with run-of-the-mill upper respiratory tract infections.

On Tuesday, the Ministry of Health said it had 1.15 million courses of Tamiflu and 50,000 courses of Relenza, enough to treat Singaporeans should the need arise.

It urged members of the public not to stockpile their own supplies. 'Indiscriminate use may ultimately limit their effectiveness as drug-resistant viruses may develop,' said the ministry in a statement.

Read the full story in Wednesday's edition of The Straits Times.