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The Genetics of Substance Abuse and Addiction

02/10/11 12:00 AM

I inherited this from my family. I was born to be a junkie like my old man and his father before him.

There’s no use going into rehab because it’s genetic.

I read that 60% of the people with genes like mine are drug addicts.

Science says that drug addiction is a hereditary brain disease so you’re stuck with it forever.

Many people addicted to drugs (or who love someone who is) become troubled when they think about the influence of genetics on substance abuse. The latest research about the human genome makes them feel doomed to lives they don’t want for themselves or their children.

However, if you think this way, it means you don’t get the full picture. The truth is that although the amount of research in this field is exploding, we still do not understand completely how genetics influences substance abuse.

We do know that you do not inherit substance abuse, you inherit a susceptibility to it. There is a big difference between the two. Even if you carry the genes for it, you cannot become a drug addict in an environment where drugs are not available. For example, the daughter of Amish farmers may never be exposed to drugs and still carry the genotypes. The son of middle-class suburbanites with similar genes may never touch drugs unless he enters a drug-friendly environment, such as a college campus or military base. In environments where peers and attitudes are drug-tolerant, he could quickly develop a problem when he experiments with drugs.

This means that genetics and environments can cancel each other out when it comes to drug addiction. People with a genetic susceptibility to drug abuse can never develop a problem, and people without the gene can literally "rewire" their brains by abusing drugs excessively. Some studies on laboratory animals indicate that certain drugs, particularly cocaine, can alter genetics so that future offspring will carry genotypes for drug susceptibility.

Scientists know more about the influence of genetics on alcoholism compared to what they know about its influence on substance abuse. For one thing, it is hard for them to study the two separately because one-half the people who use drugs also abuse alcohol. However, many experts believe that common genotypes contribute to both. The new thinking is that there are "gene clusters" that work together to create a susceptibility to substance abuse. These include genes for anxiety and depression disorders, attention deficit/hyperactivity disorder (ADHD), and most frequently, conduct disorder. In one study, 40% of children who experienced depression before they were 14 years old become substance abusers. In a study of 118 identical twins, if a twin had certain personality traits, especially thrill-seeking, he was more likely to use drugs than his twin without the trait.

Males with conduct disorders are at high risk for substance abuse, and children with ADHD are at high risk for developing conduct disorders. This does not mean that ADHD causes drug addiction. What it might imply is that if a male has a gene cluster for ADHD, depression, thrill-seeking, and alcohol abuse, he is more likely to develop substance abuse disorder.

Other factors that are not necessarily genetic also put a person at higher risk for drug abuse. These may be a tolerant attitude toward drugs, bullying or peer rejection in elementary school, problems with legal authorities, academic failure, dysfunctional family, and more. Using smoked or injected drugs is also a risk factor. All these factors complicate the science of genetic influence.

People who start using drugs early, in middle school, for example, are more likely to become addicts. Identical twins separated at birth tell us that environment (including parents) influences the age you start experimenting with substances, but genetics influences how much you use once you start.

Most new research does point to a strong genetic influence on substance abuse disorder. Scientists can breed animals with drug-seeking behaviors, for example. Adopted children tend to be more similar to their biological parents, not their adoptive ones, when it comes to drug abuse. Genetics plays a strong role in your first reactions to drugs or alcohol. Some people will get nausea from just one drink, for example. Others need three or four drinks just to get a "buzz," and these differences are about genetics and the way you metabolize alcohol.

Certain studies of identical twins brought up in different environments found 66% inheritability factor for alcoholism and 33% for drugs. Some people interpret this as meaning if you have certain genes, you have a 66% chance for developing alcoholism, but this is not the case.

A scientist may have a sample of 10,000 people, and 500 of them are substance abusers. Of this group, there are nine sets of identical twins or 18 people. In six sets of these twins, both are alcoholics, but there are three sets of twins where one is alcoholic and one isn’t. This is where our scientist gets the 66% inheritability figure. The inheritability for blue eyes is 100%, meaning all twins have the same eye color. The inheritability for substance abuse is 32%. But you have to remember that our scientist has thrown out 9,500 people who are not substance abusers, and some of whom may carry the gene for susceptibility to drug abuse. So the 33% inheritability for drug abuse is only among the population of drug abusers.

Genetics no doubt plays a strong role in determining who becomes a substance abuser. If you carry these genotypes, it means you may have to be careful in how you approach recreational and pharmaceutical chemicals in our society. By no means does it condemn you to a life of addiction.


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