I think I've a bit of a problem with this article from the APA Psychport page. The article, titled "Prescription drugs a gateway for teen drug abuse: With many substances harder to find, study shows drop in illegal drug abuse," suggests that for many teenagers nonmedical use of prescription drugs is their first drug use. I've no problem with the data in support of that claim (which is available for free here). Indeed, the data show that, as the Psychport article accurately states:
About 2.5 million new teen substance abusers were initiated through prescription drugs. Next was marijuana, with 2.1 million new adolescent users.
My concern is the use of the term "gateway drug" in the Psychport article's title. I'm not sure that an "initiation drug" is quite the same as a "gateway drug." The original data source describes "initiation" as:
Information on substance use initiation, also known as incidence or first-time use...
In this sense, initiation deals with first-time drug use. If you google "gateway drug" though, you'll find the term isn't generally used to indicate first-time usage, but rather more commonly refers to the gateway drug theory, described by The Encyclopedia of Public Health as follows:
The "gateway drug theory" describes the phenomenon in which an introduction to drug-using behavior through the use of tobacco, alcohol, or marijuana is related to subsequent use of other illicit drugs.
The data, though, provide no evidence that nonmedical prescription drug use "is related to subsequent use of other illicit drugs." Appears to me that the use of the term gateway drug is inappropriate, then...although it might make for excellent drug law propaganda given that most people don't pay much attention.