Like it or not, marijuana use has increased exponentially since President Nixon declared a war no drugs in 1971. Today, marijuana — or weed, pot, cannabis, Mary Jane — is the third most popular recreational drug in the United States, behind only alcohol and tobacco. Upward of 24 million people have used it, based on the latest estimates, with 14 million using it regularly. But despite a growing warmth toward the drug, and two states (Washington and Colorado) legalizing its recreational use, there are still some people on the fence about its safety and usefulness. So, to educate you nonbelievers...