Doctors should be cautious when prescribing the antiviral drug Tamiflu to prevent illness in people exposed to the H1N1 virus, said an expert who analyzed North America's first reported case of a Tamiflu-resistant H1N1 virus.

People who've been exposed to the H1N1 influenza A virus causing the current swine flu pandemic might already be in the process of becoming ill and that might foster the emergence of Tamiflu-resistant viruses, Dr. Guy Boivin said Wednesday.

As of late last week, the World Health Organization was reporting that 42 cases of Tamiflu-resistant H1N1 viruses had been seen globally since the swine flu pandemic began in April. The case Boivin writes about in the New England Journal of Medicine occurred in June and was the third instance of Tamiflu resistance reported.

"I have to admit I would have expected more cases, considering ... the large volume of Tamiflu that has been used in the last few months, certainly more than in the last five years, probably," said Boivin, who is based at Laval University in Quebec City.

"But I'm still concerned that the same situation that has occurred with the seasonal H1N1 could be ... repeated with the pandemic H1N1."

Seasonal H1N1 viruses developed resistance to Tamiflu a couple of years ago for reasons that remain unclear.

Though it had previously been thought that the mutation needed to confer resistance to the drug would cripple any resistant viruses that emerged, that proved not to be the case. The resistant viruses spread like wildfire, virtually crowding out susceptible seasonal H1N1 viruses and rendering Tamiflu useless against them.

Public health authorities worry that if that happened with seasonal H1N1 viruses, it could happen with the pandemic swine flu virus, especially at a time when much more of the drug is being used than ever before.

Case last summer

The case Boivin reported in the medical literature was reported on last summer.

A man from Quebec who had chronic lung disease was given Tamiflu as a prophylactic after his teenage son, who had asthma, contracted H1N1 and was treated with the drug.

A prophylactic dose is one pill a day. A treatment dose is two pills a day.

The father started experiencing flu-like symptoms within about 24 hours of starting the drug, a likely signal that he was already sick. A prophylactic dose can stave off infection, but it isn't powerful enough to kill off all the viruses in an infected person's body. Those that survive are likely to be resistant to the drug.

In fact, about one-third of the cases of Tamiflu resistance that have been spotted so far have been in people who had been given the drug for prophylaxis.

"Because of that, we need to limit as much as possible the use of Tamiflu prophylaxis and instead focus on early treatment," Boivin said.

He suggested if doctors do give patients Tamiflu to prevent H1N1 infection, they should monitor them and switch them to a full treatment dose if they start experiencing symptoms.

"If a physician decides to put someone on post-exposure prophylaxis, it's up to him to tell the person that if the person develops symptoms, they need to consult [their doctor]," he said. "And if they cannot consult rapidly, they need to switch to twice-a-day dosages."

Another antiviral expert, Dr. Frederick Hayden of the University of Virginia, said he thinks an argument could be made for using a treatment dose for prophylaxis when prophylaxis is warranted.

"If I had a highly immuno-compromised host [patient] where I knew that they were unlikely to respond to the vaccine, and they had a significant risk exposure, I would probably err on the side of giving them full therapeutic doses for that particular exposure," he said.

"But I think one needs to individualize."

Hayden said another option would be to use the other available flu antiviral drug, Relenza, when it is necessary to put someone on prophylaxis. He said Relenza is probably under-utilized for this purpose.