Scientists in San Diego have made a key discovery in the fight against Alzheimer's Disease. The only problem is it is illegal for most people to use it.

THC is the active ingredient in marijuana. Researchers at Scripps Institute have found it may prevent the progression of Alzheimers.

They say it preserves an important neurotransmitter that allows the brain to function.

Alzheimer's is the leading cause of dementia among the elderly.

• MSNBC.com: Marijuana may help stave off Alzheimer’s