Some studies have focused on finding genes that predispose the person to be dependent on marijuana, cocaine, or heroin by studying genes that control a persons dopamine and opioid receptors, but no conclusive findings were reported. Other researchers found a connection between dopamine receptor genes and dependency on a substance. A potential problem with this study was that alcohol is commonly used with another substance, so the results of the study may not have been caused by dependency on a single substance. This means that multiple substances may have been contributing to the results, but the researchers suggested that further research should be done.However, there are studies that have found evidence of the influence of genes on vulnerability to substance dependence. These studies often use genotype, or the genetic information found on a persons chromosomes, and phenotype, which consists of the visible features of a person, to look at genetic patterns. One study examined the phenotype and genotype of 1,858 participants from 893 families to look at differences in three nicotinic acetylcholine receptor genes found within these individuals. The experimenters found significant connections between receptor genes for nicotine and polysubstance dependence, which indicated that differences in these genes can create the risk of being dependent on multiple substances. Psychological
A 1989 study conducted by Khantzian and Treece found that nearly 60% of their opioid-dependent sample met criteria for an Axis II diagnosis. In the same study, 93% of the sample had a comorbid disorder, implying that the comorbid disorder plays some role in the addiction.
Because alcohol was a common co-substance for nearly all of the people in the polysubstance use group, it was difficult to tell exactly which substances were affecting certain cognitive functions. The researchers found that the difference in the two groups performance levels on executive function, or higher-level cognitive processing tasks were consistently showing that the polysubstance group scored lower than the control group. In general, this meant that multiple substances negatively affected the polysubstance groups cognitive functioning. More specifically, the researchers found that the amount of cannabis and cocaine affected the verbal part of working memory, the reasoning task, and decision making, while cocaine and heroin had a similar negative effect on visual and spatial tasks, but cannabis particularly affected visual and spatial working memory. These results suggest that the combined use of cannabis, cocaine, and heroin impair more cognitive functions more severely than if used separately.Alcohols negative effects on learning, spatial abilities and memory has been shown in many studies. This raises a question: does using alcohol in combination with other substances impair cognitive functioning even more? One study decided to try to determine if people with polysubstance use who also recreationally use alcohol would display poorer performance on a verbal learning and memory test in comparison to those who consumed excessive amounts of alcohol specifically. The California Verbal Learning Test (CVLT) was used due to its ability to "quantify small changes in verbal learning and memory" by evaluating errors made during the test and the strategies used to make those errors.