A new study finds that fever medication may help flu sufferers feel better but may unexpectedly contribute to the spread of more flu virus.

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As if catching the flu weren't bad enough, researchers have grim news for influenza sufferers: Popping pills to relieve a flu-induced fever can make you more likely to pass the virus to others.

As seasonal influenza rages across the USA, a new study reports that those who take anti-fever medications release more influenza virus into the environment than people who forgo drugs. The impact isn't negligible. Use of anti-fever drugs, technically known as anti-pyretics, raises the number of flu cases by roughly 5% a year, translating to more than 1,000 additional deaths in the USA annually for a garden-variety flu strain, the study estimates.

"The (message) of this paper is not, 'Don't take anti-pyretics.' That's not what we're saying," says study leader David Earn, an applied mathematician at Canada's McMaster University. But "if you're taking those drugs, there's an effect on the rest of the population that you should be aware of."

The unintended consequences of anti-fever drugs have been known for decades. A study in the 1970s found that people who took aspirin gave off more of a cold-causing virus than those who didn't, while a 1980s study showed that taking acetaminophen, the main ingredient in Tylenol, lengthened the infectious period for children with chickenpox. Other studies help explain these findings: Immune-system chemicals that fight infectious viruses are made in greater quantities at higher temperatures.

Few researchers, if any, have looked at the public health implications of these seemingly innocent drugs, which are widely recommended by doctors. To understand those implications, Earn and his colleagues estimated a host of factors, such as the amount of extra virus people give off – in coughs, sneezes and the like — because of fever suppression. They found that use of fever-reducing drugs is so common that it leads to a rise of 1% or so in the transmission of the flu virus during a typical year. That leads to more cases of flu and more deaths, the researchers report in this week's edition of Proceedings of the Royal Society B, a British journal.

The death number is a rough first stab, but it could very well be an underestimate. That's because the study examines only the effect of the higher levels of virus that the sick spread. The researchers didn't measure the impact of the longer period of infectiousness in patients taking anti-fever drugs. Nor does the study take into account that when people feel better after taking medications, they're more likely to resume their normal lives than if they let the fever run its course.

Earn emphasizes that flu victims should decide for themselves, preferably with the help of a doctor, whether to take medications. He urges anyone who does take anti-fever drugs to stay home and avoid close contact with others.

"Don't go to work or school because you think you're feeling better," he says. "You might spread (the virus) less if you didn't make yourself feel better and you stayed in bed with a high fever."

Other researchers say the study is intriguing but very preliminary.

The paper is based on "all these hypothetical numbers … that are not quite backed up with data," says Gerardo Chowell-Puente, a mathematical epidemiologist at Arizona State University. "So it has to be interpreted with caution."

"The paper is of interest, but more in terms of theory than practice," the University of Michigan's Arnold Monto, who studies the epidemiology of infectious disease, says via e-mail. "The effect is hard to measure."

The researchers behind the new study readily agree that they need more data to compute better estimates. They're working on a study to gather more solid information, but they hope their results will at least raise awareness.

"I think most people would imagine that if they feel better, they're probably less likely to infect other people," Earn says. But if you take drugs that lower your fever, "one effect is you will become more infectious. ... So even if you feel better and feel like interacting with people, you probably shouldn't."

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