The withdrawal from the market of Tysabri (natalizumab), the multiple sclerosis (MS) therapy, will have little immediate effect on patients in the Republic.

Fewer than 20 people here are thought to have received the drug as part of ongoing research into the disease. Tysabri has not been licensed for general use by the Irish Medicines Board.

However, the news will dampen the hopes of those with the chronic neurological condition.

Trials of the new drug had focused on its combined use with a form of interferon marketed as Avonex. Scientists were hoping the combination would increase the remission rate for MS.

However, two patients have had very serious side effects from the combination therapy. One has died; the other is thought to have developed the rare condition - progressive multifocal leukoencephalopathy (PML).

This is a progressive and uniformly fatal disease of the central nervous system.

It usually affects patients whose immune systems are already compromised, such as those with HIV/Aids or people undergoing aggressive treatment for cancer. PML is caused by the activation of the JC virus, which 80 per cent of us carry in our systems in a latent form.

The finding raises the possibility that the Avonex/Tysabri combination damages the immune system in some way.

Patients on treatment with Avonex alone should not be concerned at yesterday's development. There is no evidence that patients taking Avonex or Tysabri in isolation have ever developed PML.

Approximately 3,000 patients have been treated with Tysabri in clinical trials of MS. It has also been tested on patients with Crohn's disease and rheumatoid arthritis, two other immune system disorders. The drug is given by intravenous infusion once a month.

A leading neurologist last night questioned the decision of the US Food and Drugs Administration (FDA) to approve Tysabri at a point when limited research data was available.