US officials warn that increasing infusions of Biogen Idec Inc.’s multiple sclerosis (MS) drug Tysabri raises the risk of a potentially fatal brain disorder amongst patients.

The US Food and Drug Administration (FDA) said, Tysabri co-marketed by Elan Corp. will ensure prescribing instructions for the drug are updated to warn of the higher risk with more treatment.

The warning, FDA says is based on 31 confirmed cases of progressive multi-focal leukoencephalopathy (PML), a brain infection which can prove to be fatal, with eight patients who have already died from it.

Based on the available information, FDA notes on its website that Tysabri’s clinical benefits continue to outweigh potential risks, though there was a temporary withdrawal of Tysabri from the market in 2005 due to its PML link.

While updating PML cases on doctor’s request, Biogen no longer posts the figures on its website, nor does it issue any press release.

Elan officials were not available for immediate comment.

According to FDA, a single infusion of Tysabri is given every four weeks, with 66,000 people worldwide treated with the drug since it was allowed back on the market in 2006.

The FDA announcement follows that of the European regulators, who also decreed the benefits of Tysabri, generic name natalizumab, far outweigh the risks.

Biogen plans the first clinical trial that could result in using Tysabri at earlier stages of MS, as Tysabri is generally reserved for patients with very aggressive disease or have no other options.

However, it could soon face increased competition from other MS treatments from Novartis AG and German Merck KGaA, who in a notable advance to injections and infusions of current drugs, could launch oral treatments for the disease this year.