Granite Staters for Medical Marijuana grades the Presidential contenders on their medical marijuana stances:

John Edwards gets a D+ :

Edwards has publicly stated that he would not change marijuana laws, and he favors the Justice Department's arresting patients and caregivers who defy federal law. (emphasis supplied)

....Responding to questions from Granite Staters for Medical Marijuana on July 7, 2003 on C-SPAN, Edwards reiterated his intention to set up a commission to study the therapeutic benefits of medical marijuana. When asked if he would jail seriously ill patients while his commission studies medical marijuana, Edwards responded "what'd you just say, there are raids?" However, when asked a week later on July 15, 2003 whether he would continue the current policy of jailing sick patients, he responded "the government has a responsibility to enforce the laws," echoing a comment he made six weeks earlier.

On May 29, 2003, Edwards was quoted in the San Francisco Chronicle, as saying,.....) "It's the job of the Justice Department to enforce the law as it presently exists."

Gephardt has switched positions and although he once voted against medical marijuana, he now says he'd support it. He gets a C-:

What Gephardt has said: When Granite Staters for Medical Marijuana's Linda Macia told Gephardt at a campaign stop on July 20, "I'm a patient and a medical marijuana advocate. I'm really ill, and I can't use drugs at all. I'm allergic to narcotics, I need your help. States' rights for sick people to--," Gephardt immediately said, "That's what I'm for...states' rights." When asked if he would sign federal legislation to allow seriously ill people to use medical marijuana with their doctors' approval, he responded, "Sure."

Note: We haven't seen a transcript confirming the above conversation.

John Kerry has backtracked the other way, earning a C grade:

Kerry previously said he favored federal legislation to allow people with cancer, AIDS, and other serious illnesses to have medical marijuana, with their doctors' approval. However, Kerry recently retreated from that stance, saying he wants to rely on a scientific review before he makes any decisions about protecting patients.

Joe Lieberman is another candidate who has switched positions. In 1999, he opposed medical marijuana. He's better on the issue this time around, but still earns only a D+: