A study conducted in Norway provides more evidence that genetic factors may play an important role in the use of drugs such as marijuana and cocaine.

According to study chief Dr. Kenneth S. Kendler, previous studies on the role of genes in illicit drug use were conducted in countries with high rates of drug abuse, namely the United States and Australia.

“This is the first study to show the importance of genetic factors in a culture/country (Norway) with rather low rates of drug problems,” explained Kendler, who is from the Medical College of Virginia in Richmond.

“It has been suggested that in such countries, social rather than genetics factors should prove to be most important. Our results do not support that hypothesis,” he said.

Kendler’s team interviewed and assessed 1,386 Norwegian pairs of young adult twins about their lifetime use of illicit drugs, including marijuana, stimulants, opiates, cocaine, and psychedelics. They report their findings in the journal Psychological Medicine.

The findings overall, Kendler told Reuters Health, provide “rather strong evidence that individuals differ in their risk for drug abuse, that this difference is rather strongly influenced by genetic factors and this pattern of findings occurs in cultures with both low and high rates of drug abuse.”

As expected in this Norwegian population, “significant lifetime use” of illicit drugs, defined as use 10 or more times, was relatively uncommon, being reported by 6.4 percent of the total sample.

The tendency for twins to both take up illicit drug use was much higher in identical twins than in fraternal twins. A diagnosis of psychoactive substance use disorder in both twins, although rare, was also much more common in identical twins than in fraternal twins.

Identical twins are monozygotic, coming from the same egg that splits after fertilization, and they share the same genetic makeup. Fraternal twins are dizygotic, meaning two eggs were fertilized simultaneously, and they are no more genetically alike than non-twin siblings.

Estimates of the “heritability” of illicit drug abuse were high, ranging from 58 to 81 percent, the authors report.

“In addition to prior findings, the results of this investigation indicate that genetic factors are likely to be important risk factors for psychoactive drug use and misuse in many parts of the world,” Kendler concluded in a press release.

SOURCE: Psychological Medicine, July 2006.