pot has been the “gateway drug” for several decades. hard to dispute all those rehab center reports. Sure it is. But it is not necessarily adulterated with "more addictive" drugs. It is a gateway drug in itself. People try it, like it, and decide to try something stronger. The notion that "all street pot is adulterated" is unprovable and is a stalking horse for pot legalization.
The gateway claim is nonsense - research shows that the correlation between earlier marijuana use and later use of other drugs can be explained by a "common-factor" model, that is, a third factor that causes both results, such as individuals' opportunities and unique propensities to use drugs, or more broadly a social or psychological predisposition towards anti-social behavior. (http://www.rand.org/pubs/research_briefs/RB6010/index1.html, http://rds.homeoffice.gov.uk/rds/pdfs2/hors253.pdf)
Also note that the same sort of correlation between earlier marijuana and later harder drugs also exists between earlier alcohol and tobacco and later illegal drugs - so if marijuana is a "gateway" so are alcohol and tobacco.