The World Health Organisation says the first instance of swine flu resistant to the antiviral drug Tamiflu represents an isolated case.
The WHO says the discovery of a patient in Denmark whose infection did not respond to the drug does not amplify the severity of the virus.
The case was revealed by the Swiss drug company Roche and Danish officials on Monday.
The agency says everyone must remain alert to the virus mutating at any time.
Officials say the patient is now well.