ZURICH, March 23 (Reuters) - Concerns about the safety of Tamiflu are not affecting stockpiles of an influenza drug which would be used in a potential pandemic, a World Health Organisation (WHO) spokesman said.

Health officials widely see Tamiflu as effective in treating the H5N1 bird flu strain if given early enough. The WHO and some national governments have been stockpiling the drug in case the strain, now mainly affecting poultry, mutates and begins to spread quickly among humans.

WHO spokesman Dick Thompson said more work needed to be done to understand any link between Tamiflu -- manufactured by Switzerland's Roche <ROG.VX> -- and a series of incidents in Japan, including teen suicides, which have fuelled concerns the drug could induce psychiatric symptoms.

Roche said earlier this week that new studies from Japan and the United States showed there was no established causal link between psychiatric problems and Tamiflu. The firm says influenza can cause psychiatric symptoms such as hallucinations.

Japan's government warning that Tamiflu should not be given to teenagers sparked a clash of opinions, with some calling it too late and others saying the drug's benefits outweighed possible risks.

"Japan itself has reiterated that it's not going to change its policy on stockpiling oseltamivir as a pandemic preparedness measure," Thompson said, using the generic name for Tamiflu.

"As we understand it there needs to be some more work to understand the link, if there is one. Right now the reports seem to be anecdotal, but in terms of pandemic preparedness we don't envisage any change at this moment," he said.

There is no commercially available vaccine for the H5N1 influenza strain, which has killed at least 169 people around the world since the disease re-emerged in Asia in 2003.

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