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 is a cause of [using] those other drugs." source WebMD

The Gateway Drug Theory suggests that licit drugs, such as tobacco and alcohol, serve as a “gateway” toward the use of other, illicit drugs. However, there remains some discrepancy regarding which drug—alcohol, tobacco, or even marijuana—serves as the initial “gateway” drug subsequently leading to the use of illicit drugs such as cocaine and heroin. The purpose of this investigation was to determine which drug (alcohol, tobacco, or marijuana) was the actual “gateway” drug leading to additional substance use among a nationally representative sample of high school seniors.