Vietnam, where bird flu has killed 42 people, has banned the sale of anti-viral Tamiflu to avoid overuse of the drug.
Nguyen Duc Bon, deputy chief administrator at the Health Ministry's Pharmaceutical Control Department, said that if people who were not infected by the H5N1 poultry virus but had flu-like symptoms took Tamiflu, the drug would not be effective.
"Once they take it, the virus that infected them could mutate to be a new type that is resistant to the drug," he said.
The Health Ministry has said Tamiflu must be taken only on prescription to avoid side effects such as respiratory difficulties.
People rushed to drugstores to buy the capsules manufactured by Swiss drug maker Roche after the virus killed a man in Hanoi in late October at the start of the latest outbreaks.
Tamiflu is considered the first line of defence against the H5N1 virus that experts fear could spark an outbreak among humans if it mutates to allow human-to-human transmission.
The drug can reduce the severity of influenza and could slow the spread of a pandemic.
The H5N1 virus, which thrives best in cooler temperatures, has infected 134 people in Asia since late 2003 and killed almost 70. Vietnam has had 93 human infections, 42 of them fatal.