GENEVA - The World Health Organisation said Friday that anti-viral drugs should not be used to prevent the H1N1 flu, noting that almost half of drug resistance cases were linked to use of the medication before infection.

"The risk of resistance is... considered higher in people who receive oseltamivir for so-called 'post-exposure prophylaxis' following exposure to another person with influenza and who then develop illness despite taking oseltamivir," said the UN agency in a statement.

Oseltamivir is more commonly known as Tamiflu, and is the drug recommended by the WHO in H1N1 flu treatments.

The UN agency said that 12 out of 28 cases of drug resistance to Tamiflu has been linked to prophylactic use.

Six other resistance cases occurred in patients with severe immunosuppression and four occurred in those who were using Tamiflu as a treatment.

A further two resistance cases were identified in people who were neither using Tamiflu for prophylactic nor treatment, said the WHO, which added that those who show resistance to Roche's Tamiflu should then take an alternative anti-viral, GlaxoSmithKline's Relenza.

Overall, the number of drug-resistance cases were "sporadic and infrequent," said the WHO.

"In general, WHO does not recommend the use of anti-viral drugs for prophylactic purposes," it added.

"For people who have had exposure to an infected person and are higher risk of developing severe or complicated illness, an alternative option is close monitoring for symptoms, followed by prompt early anti-viral treatment should symptoms develop," it said.