Shares in Elan, an Irish drug group, were up 21 percent by early afternoon Wednesday based on the news that its multiple sclerosis drug, Tysabri, may be able to go back on the market after its suspension six weeks ago.

Shares in the U.S. company Biogen, which developed the drug with Elan, also rose by 1 percent on the news in after hours trading on Tuesday. Data from a recent clinical trial showed that Tysabri had reduced the risk of disability progress by 42 percent and the rate of relapse by 67 percent in 942 patients in the trials.

Some analysts doubt, however, that the drug can return to the market after the three occurrences of a potentially fatal side effect and the death of one of the patients, which spurred Tysabri’s withdrawal from the market.

Two patients contracted a rare central nervous system condition initially, and one of the died as a result of it. Additionally, another patient was identified with the same condition in March.

Still, Elan and Biogen will discuss the successful results of the trial with regulatory agencies in hopes that the benefits will be seen to outweigh the risks of the drug so that it can be returned to the market.

Elan’s shares lost 86 percent of their value in the wake of the drug’s withdrawal from sales.