Today is 4/20. Google will probably not be signaling this particular day of observance with a medical marijuana Doodle. Nonetheless, for many, 4/20 is cause for celebration.

You may be wondering how April 20 ever came to be associated with smoking marijuana. It began in 1971 in San Rafael, CA, when a group of teenagers began meeting at 4:20 PM by a statue of Louis Pasteur on their high school campus. Their code word for meeting started off as "4:20 Louis" and, over time, it became shortened to simply "420."

Thanks to a little help from The Grateful Dead and others, the code word caught on and spread throughout the counterculture.

Happy weed day.

Maybe the best way to celebrate 420 is by bringing yourself up to speed on the state of the art of medical marijuana research, and the latest in the legality of medical marijuana in each state.

Medical Uses of Marijuana: The Science

Although marijuana has been used (recreationally and therapeutically) for thousands of years, only in recent years has it received significant scientific scrutiny. In fact, it is one of the most heavily studied therapeutic substances of all time. Over 20,000 studies investigating the therapeutic effects of marijuana have been published, one third of which were published in the last three years.

Additionally, since 2005, researchers have performed 37 controlled clinical medical marijuana studies, harnessing the cooperation of more than 2,563 participating subjects.

Because marijuana is a Schedule 1 controlled substance, the use of marijuana is illegal, except for medical purposes in certain states. However, scientific consensus offers no indication that marijuana, used therapeutically, is necessarily dangerous.

In 1993, a publication prepared for the World Health Organization famously declared that, "There are no recorded cases of overdose fatalities attributed to cannabis, and the estimated lethal dose for humans extrapolated from animal studies is so high that it cannot be achieved by ... users."

In 2008, researchers at McGill University Health Centre and McGill University in Montreal, as well the University of British Columbia in Vancouver, reviewed 23 clinical studies of medical cannabinoid drugs and eight observational studies, together spanning the years 1966 to 2007. The researchers concluded that investigators "did not find a higher incidence rate of serious adverse events associated with medical cannabinoid use" than people who did not use marijuana.

In scientific communities, medical marijuana used to be viewed as a means of temporarily alleviating the pain of those suffering chronic conditions. Recent research, however, has reshaped the picture, and medical marijuana is now understood as able to modify the disease state itself.

Some of the most promising studies suggest that medical marijuana may be able to play a healing role in autoimmune disease, and as a cancer-fighting agent. For instance, recent studies have suggested how cannabinoids can be used to moderate autoimmune disorders like multiple sclerosis, inflammatory bowel disease, and rheumatoid arthritis, among others.

Marijuana in 2013: Policy and Legality

Promising findings such as these have helped push medical marijuana into the mainstream. In 2009, the American Medical Association declared for the first time "that marijuana's status as a federal Schedule 1 controlled substance be reviewed with the goal of facilitating the conduct of clinical research and development of cannabinoid-based medicines."

However, despite mounting research that cannabis may offer patients with certain conditions therapeutic benefits, only 18 states, along with Washington, D.C., have enacted laws to legalize medical marijuana.

Medical marijuana is currently legal in:

• Alaska
• Arizona
• California
• Colorado
• Connecticut
• DC
• Delaware
• Hawaii
• Maine
• Massachusetts
• Michigan
• Montana
• Nevada
• New Jersey
• New Mexico
• Oregon
• Rhode Island
• Vermont
• Washington.

As of April, 2013, ten states have legislation pending that would legalize medical marijuana.

Medical marijuana is almost legal in:

• Alabama
• Florida
• Illinois
• Iowa
• Kansas
• Kentucky
• Missouri
• New Hampshire
• New York
• West Virginia

In addition, four states have legislation pending that is favorable toward medical marijuana, but would not legalize its use:

• Maryland
• Minnesota
• Oklahoma
• Texas

It may be only a matter of time before medical marijuana is legal everywhere in the United States, especially if research continues to indicate that cannabinoids offer therapeutic benefits for some of the most common and problematic human diseases.

For those interested in the future of medical marijuana research and legislation, such prospects are worth thinking about. Maybe later today, sometime around 4:20 PM.