New findings from a group of Spanish and American researchers indicate that nearly one-half of all people who use marijuana\/cannabis will eventually transition into additional forms of drug use. Marijuana\/cannabis use is increasingly acceptable in American society, and this acceptance may lead to a significant underestimation of the short- and long-term harms associated with the intake of the drug. In a study published in February 2015 in the International Journal of Drug Policy, researchers from Columbia University and Spain\u2019s University of Oviedo used a long-term project to help determine how often people who initially consume only marijuana\/cannabis will eventually branch out into the consumption of other illicit or illegal drugs or medications. These researchers also identified some of the key factors that drive a transition from marijuana\/cannabis intake to a wider pattern of drug use. Marijuana Use in the U.S. In 2013 (the last year with fully available nationwide figures), roughly 19.8 million Americans consumed marijuana in a typical 30-day timeframe. This number was up sharply from the approximately 14.4 million Americans who consumed marijuana in a typical 30-day timeframe in 2007. Crucially, the observed increase in marijuana use contrasts starkly with the generally declining or flat usage trends associated with alcohol intake, nicotine\/tobacco intake and several other forms of substance intake. Broadly speaking, young people in their late teens or 20s have significantly higher chances of consuming marijuana\/cannabis than younger teenagers, preteens or adults over the age of 29. Despite its relatively benign reputation, marijuana ranks behind only alcohol and nicotine\/tobacco as a source of substance addiction and\/or non-dependent substance abuse across the U.S. In the context of marijuana\/cannabis intake, all issues of abuse and addiction fall under the heading of a diagnosable condition called cannabis use disorder. According to figures compiled by the National Institute on Drug Abuse, roughly 4.3 million people in the U.S. met the criteria for a cannabis use disorder diagnosis in 2012. This number far outstrips the number of people affected by diagnosable problems with opioid medications or cocaine. Initiation of Marijuana Intake Marijuana is the first drug used by more than 70 percent of all consumers of illicit\/illegal substances in the U.S. In stark contrast, only 17 percent of all illicit\/illegal drug consumers start out with opioid medications, the next most popular option for initiation of intake. In 2013, the average age for the first use of marijuana was 18. However, even children as young as 12 and 13 typically chose the drug over other potential options. Roughly 4.8 percent of all 12- to 17-year-olds in the U.S. began using marijuana in 2013. Transition to Other Drugs In the study published in the International Journal of Drug Policy, the Columbia University and University of Oviedo researchers used information drawn from 6,624 participants in an American project called the National Epidemiologic Survey on Alcohol and Related Conditions (NESARC) to estimate how often people who use only marijuana\/cannabis transition into the use of other drugs or illicitly obtained medications. The researchers used information from the same project to identify likely predictors of additional drug\/medication consumption in cannabis users. All of the participants culled from NESARC were known to have consumed marijuana\/cannabis before ever consuming another illicit\/illegal drug or medication. After analyzing the data, the researchers concluded that 44.7 percent of all people who start out as marijuana\/cannabis consumers will ultimately go on to use other illicit\/illegal substances. They also concluded that several factors increase the odds that a person who uses only marijuana\/cannabis will develop a wider pattern of drug\/medication intake. Examples of the identified factors include having \u201cexternalizing\u201d mental health problems such as conduct disorder, oppositional defiant disorder or attention-deficit hyperactivity disorder (ADHD); having \u201cinternalizing\u201d mental health problems such as depression or an anxiety disorder; and having relatively severe problems associated with cannabis use disorder. The study\u2019s authors believe that their findings add support to the \u201cgateway effect\u201d theory that describes a trajectory from marijuana\/cannabis use to the later consumption of other mind-altering, addictive substances. They also note the clear role that a range of mental health problems play in making any individual who starts out with cannabis use relatively susceptible to the initiation of additional forms of drug intake. The authors also note the key role that limitation of marijuana\/cannabis intake likely plays in the prevention of other forms of substance use.