When studying the relationship between the use of different drugs, researchers have asked whether using one substance leads to the use of others, and whether there are specific pathways through which this happens.
The question is particularly relevant from a legal and policy perspective: whether a drug that appears harmless or at least “tolerable” may nevertheless be risky because it leads to the use of other drugs, or makes people more vulnerable to developing an addiction to more dangerous substances.
Does a “gateway drug” exist—one that, in people who have used it frequently, facilitates the transition to and connection with more dangerous drugs?
The question has been raised particularly in relation to cannabis. Although illegal but widely tolerated, cannabis has been described as a “first step” toward the use of harder drugs, and as a kind of prior “brain experience” that facilitates addiction to more dangerous substances.
In other words, a drug that may not be particularly harmful or demanding in itself could nevertheless become problematic if, because of its apparent harmlessness, it becomes widely accepted and creates a “bridge” that makes the transition to harder drugs earlier and faster, pushing the person more rapidly toward addiction.
It is not enough to point out, for example, that almost all heroin addicts previously began with “lighter” drugs. The same would be true of cannabis and tobacco.
After all, among all those who try cannabis, only a minority subsequently move on to harder drugs, particularly heroin. It is true that people who use cannabis generally experiment with other drugs more frequently and at an earlier age. It is also true that people who try cannabis before the age of twenty are much more likely to move on to other drugs than those who first try it at a later age.
However, these data do not necessarily demonstrate anything, because the situation could simply be the following: people who use substances at an early age, starting with those that are legal and more readily available, may have a general tendency to experiment indiscriminately with different drugs. They will therefore be more likely to try illegal and harder drugs, and to do so at an earlier age.
In this case, the “bridge” would not be created by the substance—cannabis or anything else—but by the temperament of the person who experiments and takes risks, and who is attracted to anything capable of stimulating or altering their state of mind. This is sometimes referred to as a “drug-seeking” or “toxicophilic” personality.
A study by the World Health Organization (Degenhardt, 2010) provides a clearer finding: changes in the prevalence of nicotine use resulting from restrictive laws in the United States did not affect the use of drugs that had previously followed nicotine in the sequence.
It therefore appears that people who tend to use drugs may begin with a “lighter” substance and then move on to a more dangerous one, but without necessarily choosing a specific drug as the next step.
Important predictors of the risk of drug abuse in adulthood include:
use of any drug;
early age at initiation of any drug use;
the presence of a mental disorder before the age of 15.
It therefore does not appear that there are specific “gateway drugs,” but rather “gateway temperaments”: people move from one drug to another more often than drugs themselves create bridges that lead people from one substance to another.
Biological “bridges”
There may, however, be another sense in which “bridges” exist. Some drugs may create a biological predisposition that shortens the time required to develop an addictive relationship when the person subsequently switches to a different substance.
This appears to be the case with heroin in relation to alcohol. This does not mean that alcoholism is a consequence of heroin addiction in everyone who has used heroin—in fact, alcohol dependence is much more common—but among people who have used heroin, susceptibility to the mind-altering effects of alcohol appears to be more explosive, as though their brains had already been prepared and were “hungry” for an effect of this kind.
In this way, someone who begins using alcohol after having previously been addicted to heroin may become attached to it more intensely and progress toward dependence more rapidly.
From stimulants to sedatives
Another type of “bridge,” which seems to apply to many addiction trajectories, is the progression from stimulant drugs to sedative drugs.
People who begin by using amphetamines, cocaine, or psychedelics tend over time to move toward mixed substance use, in an attempt to compensate for the anxiety and depressive effects produced by stimulants. Later, they may progress toward predominantly sedative use.
Over time, the brain changes, and the effects of the initial substances create a vulnerable terrain on which subsequent substances can become established.
The “bridge” is represented by the psychotoxic effects of the initial substances, which make substances such as alcohol, heroin, and tranquilizers particularly appealing to brains that can no longer tolerate pure stimulation or an agitated form of euphoria, but instead seek a calmer form of euphoria—a state of absence and narcosis rather than excitement and hyperactivity.