A new US study has found that the risk of a potentially fatal brain disorder increases as patients recieve more infusions of Elan's multiple sclerosis drug Tsyabri.
The drug, which the Irish pharmaceutical firm Elan co-produces with Biogen, was withdrawn from use for a time two years ago after it was first linked to the brain disorder PML.
This latest warning is based on 31 confirmed cases of the progressive infection.
Elan's share price fell 0.6 percent on the New York Stock Exchange in response to the news.