According to WebMD...

Marijuana May Slow Alzheimer's

Key Marijuana Compound Beats Current Alzheimer's Drugs in Test-Tube Study

By Daniel J. DeNoon
WebMD Medical News
Reviewed by Louise Chang, MD

Oct. 6, 2006 -- THC, the key compound in marijuana, may also be the key to new drugs for Alzheimer's disease.
That's because the marijuana compound blocks the formation of brain-clogging Alzheimer's plaques better than current Alzheimer's drugs.
The finding -- in test-tube studies -- comes from the lab of Kim Janda, PhD, director of the Worm Institute of Research and Medicine at Scripps Research Institute.
"While we are certainly not advocating the use of illegal drugs, these findings offer convincing evidence that THC possesses remarkable inhibitory qualities, especially when compared to [Alzheimer's drugs] currently available to patients," Janda says in a news release.