States who oppose marijuana legalization frequently cite the correlation between "innocent" marijuana use, and more serious drug offenses, arguing that marijuana use is correlated with increased probability of moving on to more significant drug use later in life. Leading research proves that marijuana does not cause people to use hard drugs and asserts that this gateway theory presents the statistic association between common and uncommon drugs as a causal relationship rather than a correlation. The relationship between marijuana and harder drugs changes over time as different drugs become more or less popular. Because marijuana is the most popular illegal drug in the U.S. today, people who have used less popular drugs (heroin, cocaine, LSD) are also likely to have used marijuana. Most marijuana users have never used any other illegal drugs (Morral 2002, Kandel and Davies, 1992).
Opponents also argue that marijuana can cause permanent mental illness. However, there is no convincing scientific evidence that marijuana causes psychological damage. Some users may experience distress following ingestion, and while these experiences can be frightening, the effects are temporary. Large doses can cause temporary toxic psychosis, but this generally occurs rarely and almost always when eaten rather than smoked (Iverson 2005, Weiser and Noy 2005, Andreasson 1987, Degenhardt 2003, Weil 1970).
Opponents further argue that marijuana is highly addictive, leading long-term users to experience dependence and withdrawal. However, research indicates that most people who smoke use it only occasionally. Less than 1% smoke on a daily basis, and an even smaller minority develop either psychological or physical dependence (Johnson 1996, Kandel et al. 1997, Stephens et al. 1993). Opponents also argue that the drug has become more potent today in the past, therefore today's youth are using a much more dangerous drug. However, it is important to remember that when today's youth use marijuana, they are using the same drug that was used in the 1960s and 1970s. A small number of low-THC samples were seized by the Drug Enforcement Administration and found a dramatic increase in potency. These samples were not representative of the marijuana generally available to users during this area. Potency data from the early 1980s is more reliable and shows