Now that the dust has settled on the sudden withdrawal by Elan of its new Multiple Sclerosis (MS) treatment, Tysabri, the question in many patients and investors minds is: will the regulatory authorities allow the drug be remarketed in the future?

Although Tysabri has not yet been approved for use in Europe, it was given to 12 patients attending St Vincent's Hospital, Dublin, as part of an ongoing international trial into its efficacy and safety.

However, the MS treatment had been fast-tracked for approval by the Food and Drug Administration (FDA) in the US.

The approval last November, following what a number of neurologists felt was limited phase 3 trial data, was justified by the FDA because of a 66 per cent reduction in the number of relapses suffered by MS patients given Tysabri.

By medical research standards, this was a very significant finding in a disease where one of the key aims of treatment is to limit the number of debilitating relapses and to increase the length of time a patient spends in remission.

However, the number of patients in the studies that reached this conclusion was relatively small.

This has raised fears that the improvements attributed to the drug may not be so impressive when data from larger-scale trials becomes available. And it highlights the ongoing tension between drug companies and regulatory authorities as to how long a drug needs to be studied if rare but potentially fatal side-effects are to come to light.

On a positive note, there is currently no evidence to suggest that Tysabri, when used alone, causes any side-effects other than those listed on its data sheet.

It is known to increase the risk of infection, especially pneumonia, as well as causing skin and hypersensitivity reactions.

But none of these would be unusual, or indeed unacceptable, to a specialist treating patients with MS as long as larger studies were carried out to verify that they occurred at an acceptably low level.

So what now for Tysabri? Undoubtedly Elan will seek to present it to the FDA for reapproval as a drug to be used as monotherapy. Indeed, it could be argued that if Elan and Biogen had not gone for the "holy grail" of combining the new drug with existing interferon therapies, then Tysabri could still be on the market and the two US patients affected by the combination would now be alive and well.

In any event, the FDA and Elan will be negotiating Tysabri's return to the market. The regulators, if they are cautious, will seek a lot more data about its use in a much larger number of patients.

Elan will argue that, as a monotherapy, it has already been accepted as both safe and effective.

How long will all this take? It recently took the FDA just six months to give a group of arthritis drugs (Cox 2 Inhibitors) the all-clear, after it was found they at least doubled the risk of heart disease in people who took them for long periods.

Six months is the least the decision should take. Instead of the MS drug being back on the market in the final quarter of this year, it may well be 2006 before it completes its rehabilitation and is available for patient use again.