Progressive multifocal leukoencephalopathy (a rare brain infection). Vasogenic edema (fluid buildup in the brain). Elan (NYSE:ELN) sure hasn't had the best luck with side effects caused by its drugs lately.

Yesterday, Elan and its partner Wyeth (NYSE:WYE) said they're dropping the highest dose of their Alzheimer's treatment, bapineuzumab, in two phase 3 trials, because the dose seems to be causing a buildup of fluid in the brain.

Dropping the highest dose in a phase 3 trial is rarely a good thing. There's a reason why drugmakers choose the doses that they decide to use in phase 3 trials: The highest dose almost always has the highest effectiveness -- and often the worst level of side effects. The worst-case scenario is that bapineuzumab turns in the same lackluster performance that Human Genome Sciences' (NASDAQ:HGSI) hepatitis C drug, Albuferon, did after the drugmaker had to drop the highest dose because of safety concerns.

Bapineuzumab does have one advantage: Current Alzheimer's treatments such as Pfizer's (NYSE:PFE) Aricept, Novartis' (NYSE:NVS) Exelon, and Forest Labs' (NYSE:FRX) Namenda don't work too well. Even reduced effectiveness with the lower dose might be enough to get the drug approved.

Aside from the news about the higher dose, investors need to settle in for a long wait for the trials altogether. The bapineuzumab trials aren't fully enrolled yet, and the treatments will last for 18 months. If you're going to own Elan now, you should be investing because you think that Elan and Biogen Idec (NASDAQ:BIIB) have a chance to give their multiple-sclerosis drug, Tysabri, a makeover. Tysabri's sales will drive Elan for the immediate future.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Novartis is a Global Gains recommendation, Pfizer is an Inside Value selection, and Biogen Idec is a Stock Advisor selection. The Fool has a disclosure policy.