Rationale: Rationale: Gateway Hypothesis (or gateway drug effect) is a theory that assumes someone who uses a “softer” drug like cannabis increases their likelihood of using more dangerous drugs. (Miller & Hurd, 2017) People turn to cannabis for recreational or medicinal purposes, but their cannabis use might increase the likelihood of them experimenting with other drugs in the future (Fergusson, 2006). This study looked at the frequency of cannabis use and using other illicit drugs. 

Method: Researchers conducted a longitudinal study that collected data from 1,265 people from birth to 25 years old. They collected information from a variety of sources; parent interviews, teacher reports, self-reports, psychometric assessments, medical and other record data. At ages 15, 16, 18, 21 and 25 the subjects were questioned about the frequency of their cannabis use since their last assessment. During these assessments subjects were also asked to provide information about any other drug use for example; stimulants, hallucinogens, and illegally obtained prescription medication or opioids  

Results: In 98% of cases, cannabis had been used either in the same year or prior to other illicit drugs. Increased frequency of cannabis use was associated with higher rates of other illicit drug use This association varied by age; at age 15 weekly cannabis users the likelihood of using other illicit drugs was 60x higher than non-users. At age 25 that risk dropped to dropped to 3.6x between weekly and non cannabis users

Meaning: Potential reasons for the association between cannabis use and eventually using other substances: 

Differential association: social factors that surround cannabis use may introduce frequent users to other forms of drug use

Biochemical factors: cannabis use may lead to changes in brain chemistry 

Individual learning: experimenting with cannabis first introduces users to the pleasurable effects of drug use thus prompting them to try other types of drugs