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Shares in Irish pharmaceutical company Elan have fallen sharply today after another patient taking its multiple sclerosis drug Tysabri was diagnosed with a potentially life-threatening brain illness progressive multifocal leukoencephalopathy (PML).

Tysabri is jointly marketed by Irish pharmaceutical firm Elan and its partner Biogen. Elan shares were trading down 16.5 per cent at €4.88 on the Dublin market by 11.25am.

The diagnosis is the third Tysabri patient diagnosed with PML since the drug's reintroduction to the market in 2006.

The drug was withdrawn in early 2005 in the US when the first cases of (PML) in patients using Tysabri emerged.

Tysabri was subsequently withdrawn from the market after three patients, two of whom died, were found to have contracted the illness. It was reapproved for usage by the US Food and Drug Administration (FDA) in the middle of 2006.

The report from Biogen last night sent its shares down as much as $6.94, or 17 per cent, to $35 in extending trading on the Nasdaq Stock Market.

In this latest case the patient was diagnosed after 14 Tysabri infusions, Biogen said last night in a statement.