of prescription stimulants. The elevated risk remains after accounting for prior substance use and other risk factors. A meta-analysis of 2018 came to the conclusion that the use of electronic cigarettes increases the risk of later use of conventional cigarettes. Associations aside from first-time use The role of cannabis use in regard to alcohol use and Alcohol Use Disorder (AUD) is still not fully understood. Some studies suggest better alcohol treatment completion for those who use cannabis, while other studies find the opposite. A recent review of 39 studies which examined the relation between cannabis use and alcohol use found that 16 studies support the idea that cannabis and alcohol are substitutes for each other, 10 studies found that they are complements, 12 found that they are neither complementary nor substitutes, and one found that they are both. A study involving self-reported data from a sample of 27,461 people examined the relationship of cannabis use and AUD. These respondents had no prior diagnosis of AUD. Of the 27,461 people, 160 had reported cannabis use within the past year. At the end of a three-year period, it found that those who had previously reported cannabis use were associated with a five times greater odds of being diagnosed with AUD than those who had not. After adjustment for select confounders (age, race, marital status, income, and education), these odds were reduced to two times greater risk. Another sample of self-reported data from 2,121 persons included only those who had already been diagnosed with AUD. In this sample, it was found that those who had reported cannabis use in the past year (416 people) were associated with 1.7 greater odds of AUD persistence three years later. After adjustment for the same confounders as before, these odds were reduced to 1.3. Causes Because a sequence of first-time use can only indicate the possibility – but not the fact – of an underlying causal relation, different theories concerning the observed trends were developed. The scientific discussion (state of 2016) is dominated by two concepts, which appear to cover almost all possible causal connections if appropriately combined. These are the theories of biological alterations in the brain due to an earlier drug use and the theory of similar attitudes across different drugs. Alterations in the brain Adolescent rats repeatedly injected with tetrahydrocannabinol increased the self-administration of heroin (results based on 11 male rats and >50 male