A study published in the American Journal of Drug and Alcohol Abuse adds even more evidence showing that marijuana use itself does not cause people to use harder drugs. HealthDay reports: "We found that marijuana use within itself wasn't a risk factor for use of other drugs," said lead author Joseph Palamar, an assistant professor in the New York University Langone Medical Center's department of population health. "People do generally use marijuana before other drugs, but that doesn't mean marijuana…