The term "designer drug" hit the national vocabulary in 1985 when the federal government banned MDMA, a speed-related drug known as "Ecstasy." Designer drugs first appeared in the late 1960s, and chemical variants of methaqualone, PCP and amphetamine have surfaced in the last decade.

Scores of deaths, most of them in California, have been directly linked to fentanyl-related designer drugs, and experts say because the synthetic is extremely difficult to detect in body fluids many more such deaths may have gone undetected.

(1988 Article) Potent New Designer Drug Seized in Federal Raid on Simi Valley Lab : https://www.latimes.com/archives/la-xpm-1988-08-04-mn-10241-story.html

Emergence of the Concept:
The concept of 'designer drugs' gained attention in the 1980s as a new expression for describing a number of recent arrivals among the psychoactive substances on the illegal drug markets. When introduced for intoxicating purposes, the substances in many cases escaped formal drug control, such as the Controlled Substances Act (US), the Misuse of Drugs Act (UK) or the Narcotics Control Act (Sweden); nor were they regulated under the United Nations International Drug Conventions (Single Convention on Narcotic Drugs, 1961; Convention on Psychotropic Substances, 1971). Thus, they were rapidly perceived by public authorities to constitute a challenge to traditional drug control.

The concept of designer drugs is generally attributed to Dr Gary L. Henderson of the University of California at Davis, who introduced the concept to describe new, untested, legal synthetic drugs mimicking the effects of illicit narcotics, hallucinogens, stimulants and depressants. The concept has, however, no established scientific or codified judicial definition. Instead, several definitions have been advanced: most converge on the following criteria, stating that designer drugs are:

1. Psychoactive substances;
2. Synthesized from readily available precursor chemicals;
3. Marketed under attractive 'trade marks' (fantasy names);
4. Not subjected to legal control as narcotic drugs or psychotropic substances, at least not at the time of their introduction on the nonmedical drug market.
   Sometimes the following criteria are added:
5. Being produced mainly or exclusively in clandestine laboratories;
6. Having little or no established medical use.

In common parlance:
A designer drug can be said to be a synthetic psychoactive substance having pharmaco-logic effects similar to controlled substances and initially remaining outside traditional drug control.

Sometimes these drugs are referred to as analogs, analog drugs or homologs:
To stress the analogy or homology of their pharmacologic properties (and chemical structure) to well-known controlled substances.

In media reporting:
The concept also has a connotation of novelty: the drugs are seen as modern creations in the same sense as designer clothes in fashion.

Some designer drugs have nevertheless been known for decades:
The process for synthesizing 3,4-methy-lenedioxymethamphetamine (MDMA) was patented in 1914.

The demarcation of the substances regarded as designer drugs is not universally accepted. Sometimes 'old timers', such as methamphetamine (widely abused in Japan in the 1950s), are included. The concept is fluid and dynamic, mirroring a continuous search for new psychoactive substances.

The pioneer in this field is the American pharmacologist and chemist Alexander Shulgin (b. 1925), who for decades has been synthesizing and investigating the effects of hundreds of such substances.

Some designer drugs have also been studied experimentally in cult-like settings in religiously flavored attempts to experience altered states of human consciousness.

Designer Drugs: http://what-when-how.com/forensic-sciences/designer-drugs/