Discussions of legalizing the use of marijuana often revolve around the contention that pot functions as a “gateway drug” — a substance that can lead individuals to abuse “harder” substances like cocaine or heroin.
That assertion, which has been raised in connection with Nevada’s Question 2 on the Nov. 8 ballot, has some support in the scientific community, but experts say a lack of definitive research on the subject leaves plenty of room for argument.
“There’s not enough evidence to prove (it) one way or another,” said Nathan Gillespie, an assistant professor of psychiatry at Virginia Commonwealth University who has done research on drug use and genetics.
Marijuana, the most commonly used illicit substance in the country, is legal in Nevada for medicinal purposes and would become legal for recreational use if voters approve Question 2, which was narrowly leading in the most recent Review-Journal poll on the measure.
Though most people who abuse substances like cocaine or opioids try marijuana first, there is no proven causal relationship between use of marijuana and other illicit drugs, according to the National Institute on Drug Abuse.
The issue from a scientific perspective is that a typical pattern of progression isn’t the same as establishing that Behavior A causes Behavior B, Gillespie explained. For example, he said, studies have shown that hard drug users also frequently drank alcohol and consumed nicotine before moving on to more dangerous and addictive drugs.
“(But) just because there’s a temporal order doesn’t mean there’s causality,” he said.
Some scientists are persuaded
A commonly cited 2015 article on marijuana’s potential “gateway” properties in The International Journal on Drug Policy, based on responses to a national survey on use of alcohol and other mind-altering substances, found that about 45 percent of adults who used cannabis at some point progressed to use of at least one other illicit drug.
The strong correlation between marijuana use and use of harder drugs found by that study and similar research has supported the belief of some scientists that the link is real.
In an April opinion piece in The New York Times titled “Marijuana Has Proven to Be a Gateway Drug,” Robert L. DuPont, president of the Institute for Behavior and Health and a past director of NIDA, argued that legalizing marijuana would expand the country’s drug abuse problem.
“Like nearly all people with substance abuse problems, most heroin users initiated their drug use early in their teens, usually beginning with alcohol and marijuana. There is ample evidence that early initiation of drug use primes the brain for enhanced later responses to other drugs,” he wrote.
A series of studies on animals have bolstered the gateway theory, finding that tetrahydrocannabinol, the active ingredient in marijuana, can increase the risk for addiction to nicotine and opiates, though other published studies have cast doubt on the opiate claim.
Critics of the gateway argument in regard to marijuana say the animal studies fall short of establishing a causal connection and more studies of marijuana’s effects on humans are needed.
They also note that similar correlations have been found with alcohol and nicotine. A 2011 journal article in Science Translational Medicine, for instance, found that nicotine also altered the brain, making it easier for users to become addicted to cocaine.
Some researchers believe environment and accessibility play primary roles in people’s progression from marijuana to harder substances, according to the NIDA website.
Instead of a gateway-type theory proposing that marijuana leads to use of harder substances, proponents of this theory believe the movement from drug to drug is predicated on outside factors like genetics or environment.
“An alternative to the gateway-drug hypothesis is that people who are more vulnerable to drug-taking are simply more likely to start with readily available substances like marijuana, tobacco or alcohol, and their subsequent social interactions with other substance users increases their chances of trying other drugs,” NIDA says.
A third theory indicates use of the drugs is a mixture of gateway effects, genetics and other factors.
Because marijuana is listed as a Schedule I controlled substance by the U.S. Drug Enforcement Agency, research involving the substance has been tightly controlled in the past. However, the DEA announced in August it would allow more approved growers to distribute the drug to authorized institutions to foster research. That, in turn, could pave the way for comprehensive studies that could put the debate to rest.
Gillespie said a lengthy study of twins could definitively answer the question.
Such a study would allow scientists to control for factors like genetics and environment, Gillespie said, and demonstrate once and for all what role marijuana use plays, if any, in leading a user down the path to addiction.
“The correct models have never been properly or adequately tested,” he said.
Contact Pashtana Usufzy at firstname.lastname@example.org. Follow @pashtana_u on Twitter.