According to the Substance Abuse and Mental Health Services Administration, in 2013 more than 24 million Americans over the age of 12 used illicit drugs and more than half the population drank alcohol(1). However, not everyone who takes drugs or drinks alcohol become addicted to their effects. For instance, just 8.5% of Americans have problems with alcohol dependency(2), so the vast majority of drinkers manage to stay in control of their alcohol intake. Similarly, less than 5% of people prescribed opiate based painkillers develop an addiction to them, even though as many as 20% abuse their medication(3). The prevalence of addiction does vary between drugs though, with close to 15% of marijuana users developing a heavy dependency(4), rising to as high as 90% among meth users(5). While certain substances are more addictive than others, why do some people become addicted and others manage to escape substance dependency? It is suggested that there is a genetic predisposition to addiction, and while there is evidence that your genes can make a difference to whether you become an addict, they are only part of the story.
Genetics and Addiction
Although the media often talks about an addiction gene, as the University of Utah explains, there is not just a single gene that controls vulnerability to addiction(6). Like the majority of other health problems, addiction susceptibility is highly complex, with a range of factors influencing the likelihood of addiction, including multiple genes. As a result, not all addicts carry the same genes and not everyone with these so-called addictive genes will develop a drink or drug habit, as environmental factors are also important.
As there is a genetic component to addiction, alcohol and drug abuse often run in families, as parents are able to pass an addictive gene on to their children. However, even if your parents are alcoholics or drug addicts, it is not inevitable that you will develop an a problem with substance dependency too, as the University of Pennsylvania discusses that addiction can skip a generation(7). This also means though that the absence of addictive habits in your parents is no guarantee of protection, particularly if other family members such as grandparents, aunts or uncles have a drink or drug problem. Genetic transmission of addiction seems to be particularly significant between certain family members though, such as between fathers with an alcohol misuse disorder and their sons, with the risk of alcohol abuse 9 times that of the population as a whole. Even then, addiction is not a certainty, as it depends on the conditions a son is exposed to during their childhood and early adult life.
Genetic Causes of Addiction
With addiction genetics can act indirectly on your risk of developing a dependency on alcohol or drugs such as opiates. For example, there is growing evidence for a genetic basis for some of the personality traits that make people more likely to abuse drink and drugs, such as being impulsive, lacking inhibitory control and preference for risk taking(8). A gene known as TPH1, which regulates serotonin production, is linked to impulsivity, which is associated with initiating substance misuse. The move to outright addiction is linked to a risk-taking personality and a change in the gene that encodes the dopamine receptor may help to explain this. Based on these findings, it seems that heavy drinking and drug use are not down to poor decisions, but are driven by our intrinsic traits.
Specific genes have also been identified, which help to explain the genetics of addiction. For instance, a variant of the gene that encodes the mu opioid receptor is linked to opiate dependence. A single alteration in the gene is common in heroin users, which causes reduced receptor function, so they need to take increasing doses of the drug to achieve a high, increasing the likelihood of addiction. Meanwhile, a gene mutation that leads to lower levels of dynorphin, which usually inhibits dopamine production, is linked to susceptibility to cocaine dependency. Alternatively, an alteration in the gene responsible for the enzyme that breaks down dopamine results in reduced enzyme activity and higher levels of dopamine, which is associated with a predisposition to dependency on multiple substances and one of the potential causes of drug abuse.
In relation to alcohol abuse, a number of genes appear to protect or contribute to alcohol dependency, including those responsible for alcohol metabolism and receptors in the brain(9). A variation in the gene for alcohol dehydrogenase, the enzyme that breaks down alcohol into less toxic substances, increases the rate at which alcohol is cleared and therefore reduce the likelihood of an alcohol misuse disorder. Alternatively, a variant of aldehyde dehydrogenase, the enzyme that controls levels of alcohol breakdown products in the blood, which results in low enzyme activity is also protective against alcohol misuse due to the adverse effects experienced after even small amounts of alcohol. Meanwhile, an alteration in the gene that encodes the GABA receptor, which is responsible for the effects of alcohol, leads to increased relaxation, alcohol tolerance and dependence. The acetylcholine receptor also mediates alcohol’s effects and one form of this gene additionally helps to explain alcoholism genetic susceptibility.
Addiction and Environment
While researchers have discovered genes controlling addictive behaviors, that only partly answers the question “Is addiction genetic?” As already mentioned, genes alone do not influence susceptibility to addiction, as environmental factors also come into play. Evidence for this comes from twin studies. By studying identical twins, who share exactly the same genetic material, it is possible to see whether addiction is completely down to genetics, as if it was both twins would share the same trait(10). However, this is not the case, as Harvard Medical School highlights that twin studies show that just 50% of addiction vulnerability is inherited(11). The other 50% is down to environmental exposures. Siblings raised by the same parents, irrespective of whether they are identical or non-identical twins, may show similar behaviors simply because of the way in which they are brought up and the other influences they come into contact with. Addiction is a family disease not simply down to shared genetics, but due to shared environments as well.
Whether a genetic predisposition to alcoholism or drug addiction leads to these habits depends on a range of risk and protective factors that you may be exposed to. With genetic susceptibility risk factors make addiction more likely, while protective factors can overcome the genetic influences and reduce the likelihood of problem behaviors. These factors can be grouped according to family, community and societal influences(12). When it comes to addiction and the family influence, exposure to parental drink and drug habits (or indeed those of other relatives), neglect, abuse and mistreatment, and poor parental supervision all increase the risk of alcohol and drug dependency. Meanwhile, close parental involvement can protect against addictive habits, whether through informing of the dangers of substance misuse or protecting against exposure to drink and drugs. In the community, factors such as living in areas of deprivation and those where violence is common are risk factors for drinking and drug taking, though involvement in community activities, such as faith membership and recreation, can offer a degree of protection. Where society is concerned, when heavy drinking and drug use is accepted, both by individuals and the law, and minority groups are marginalized, this can encourage substance abuse, whereas effective policies on substance misuse and social inclusion can reduce the likelihood of adopting risky behaviors.
Addiction Treatments Based on Genetics
Understanding the genetics of substance dependency doesn’t just help us to understand possible addiction causes more fully, but it can also aid researchers in their quest for more effective treatments for alcohol and drug abuse. Genetic variation may help to explain the differences in treatment success between addicts and therefore can be used when developing new therapies to manage substance dependency. For each potential drug addiction or alcoholism gene identified, there is another possible drug target available, allowing researchers to focus on the item coded for by the gene in question to modify its action. As a result, this may successfully change communication pathways in the brain, helping to restore normal brain function. In the future, genetic testing may also show which medications will offer an addict the best chance of quitting their habit based on their genetic profile.