The third time isn't the charm for Medivation (Nasdaq: MDVN) and Pfizer's (NYSE: PFE) Dimebon -- not that anyone thought that it was.

Nearly two years ago, Dimebon failed to show an effect in Alzheimer's disease; patients actually fared worse for some measures.

Last year, the drug failed again, this time in Huntington's disease, another neurological disease.

Back to Alzheimer's disease today, in a trial called CONCERT, with the same result. Combining Dimebon with Aricept, sold by Eisai and Pfizer, didn't help patients any more than Aricept alone. No melding symphony in this CONCERT.

Maybe Medivation and Pfizer should have ended Dimebon's development earlier, but this trial was already ongoing when the first trial failed. As I said when Dimebon failed in Huntington's disease, the CONCERT trial acted as a call option for investors with a huge upside and very little downside. Everyone was expecting it to fail; Medivation's value is entirely tied up in its prostate cancer drug MDV3100, which has seen much better success than Dimebon.

Unfortunately call options often expire worthless. So do partnerships; the companies are cutting their losses -- finally -- shutting down the Dimebon program, and going their separate ways.

For better or worse, Pfizer still has a hand in Alzheimer's disease. Bapineuzumab, its most advanced Alzheimer's drug, was acquired in its purchase of Wyeth; Johnson & Johnson (NYSE: JNJ) and Elan (NYSE: ELN) split the other half. There are higher hopes for bapineuzumab than there were for Dimebon, but only mildly so.

Alzheimer's disease is hard to treat in large part because we really don't know what causes it. Bapineuzumab is supposed to work by inhibiting the formation of amyloid plaques, the same basic theory, although through a different mechanism, as Eli Lilly's (NYSE: LLY) semagacestat, which failed to help Alzheimer's patients. Lilly has another phase 3 Alzheimer's drug in development, so it has another shot to succeed (and another shot to fail).

Failing is a theme with Alzheimer's drugs, making them appropriate only for very risk-tolerant investors or those that like the rest of the pipeline and assign zero value to the Alzheimer's programs. If you're looking for something a little more stable -- something more appropriate for a retirement account perhaps -- check out the Fool's new free report, "3 Stocks That Will Help You Retire Rich."

Fool contributor Brian Orelli holds no position in any company mentioned. Check out his holdings and a short bio. The Motley Fool owns shares of Johnson & Johnson. Motley Fool newsletter services have recommended buying shares of Pfizer, Johnson & Johnson, and Elan and creating a diagonal call position in Johnson & Johnson. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.