The Five Dumbest Things on Wall Street This Week

1. Mr. Dash

Elan (ELN Quote) was sounding a little less self-assured this week.

The Irish drugmaker has been hit hard by the safety problems of its key drug, the multiple sclerosis treatment Tysabri. Elan and partner Biogen (BIIB Quote) yanked Tysabri on Feb. 28 after a patient died of a rare brain infection. "Tysabri is probably dead," a Morgan Stanley analyst wrote in March, summing up a view widely held on Wall Street.

Questions about the drug haven't silenced CEO Kelly Martin, though. He raised eyebrows in February by saying a realistic timeline would put Tysabri back on the market in the third quarter. Then Martin pushed the stock up 11% one day last week by reiterating that he expected Tysabri to revive.

But Elan was showing a little less dash Thursday. The stock plunged 15% after The Boston Globe reported that a fourth patient using the drug may have contracted the brain disease, which is known as PML.

Thursday's report halted a monthlong run-up. It also left Elan in a less talkative mood. "As per previous statements, we don't comment on rumors or individual cases going through the safety evaluation process," Elan told Reuters and Bloomberg on Thursday.

Martin might do well to keep his trap shut altogether.