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As health officials across the globe prepare for the possibility of a global pandemic of swine flu, they have a relatively new weapon in the fight against influenza; antiviral drugs, first developed in the 1990s, have been shown to help treat and prevent influenza. But such drugs have never been used to tackle a widespread outbreak of influenza before, and there are concerns they may quickly prove ineffective.

How do antivirals work?
If administered soon after symptoms appear, antivirals Oseltamivir and Zanamivir (brand names Tamiflu and Relenza) are believed to reduce the severity and shorten the duration of the disease by current strain of swine flu. The drugs work by inhibiting an enyzme chemical helper that the flu virus uses to spread from infected human cells to healthy ones. So while not killing the virus, it helps the body fight off the disease by slowing its spread. This, in turn, may help prevent "acute respiratory distress syndrome" — the sudden worsening of flu that, along with secondary lung infections, is a main cause of death among influenza patients. There is also evidence to suggest that they can be used prophylactically — to prevent rather than treat the disease. "We don't have many tools in our medicine cabinet to fight this disease, but this is one of them," says Howard Markel, a physician and director of the History of Medicine at the University of Michigan. (See pictures of the swine flu outbreak in Mexico.)

What role can drugs play in tackling a swine flu outbreak?
Over the last few years, concern over a bird flu pandemic spurred many governments to stockpile antivirals. While the current swine flu outbreak remains limited in scope, health agencies will likely offer antivirals as a prophylaxis only to those who may have been exposed to the disease: asymptomatic passengers on the same flight as a sick Spanish man, for example, have been given Oseltamivir as a precaution. In New Zealand and Mexico, where there are confirmed cases of the disease, the drug has been made available over-the-counter, although pharmacists can exercise discretion about who they sell to. Should the outbreak turn into a global pandemic, there simply aren't enough drugs available for universal use; they will be given only to those suspected of being ill with swine flu, and to front-line healthcare and essential government workers as a prophylactic. (See five things you need to know about swine flu.)

But if they aren't used to prevent swine flu, can they help slow the spread of a pandemic?
The most effective way of slowing a pandemic is to develop a vaccine. But doing so can take months. In the interim, antivirals may play a vital roll by making ill patients less contagious. When a person is sick with the flu, he or she "sheds" virus through coughing, sneezing and other excretions. Effective antivirals lessen the amount of virus a patient sheds (because the patient is not as severely ill) and shortens the length of time he or she sheds virus at all. Taking this into account, Neil Ferguson of Imperial College in London has used mathematical modeling to show that antivirals would help slow the spread of a bird flu virus originating in Asia. While the model should hold true for a swine flu arising in Central America, as well, no one knows for sure as the drugs have never been used to tackle an epidemic or pandemic before. (See pictures of bird flu.)

Can swine flu become resistant to these drugs?
Yes, and that's another reason why health officials will want to limit their use to those who have become ill with the disease, according to Hugh Pennington, a virologist at the University of Aberdeen. Resistance occurs when a virus mutates in such a way as to render a drug ineffective. This is more likely to occur when an antiviral is widely used because resistant mutations are more likely to thrive and be passed on. A similar process has led to the widespread existence of antibiotic resistant bacteria such as MRSA. But it can also happen spontaneously: during this winter's flu season, when antivirals were not widely used, the dominant strain of influenza suddenly became resistant to Oseltamivir. Doctors are uncertain as to why. In a pandemic situation, when the drugs will be widely prescribed, many virologists believe that resistance will inevitably develop — they just hope it will happen slowly.

Read "How to Deal with Swine Flu: Heeding the Mistakes of 1976."

See TIME's Pictures of the Week.

• Eben Harrell / London
• The drugs, developed just over a decade ago, will be one of the main weapons in the fight against Swine Flu, but they have limitations