Methaqualone is a depressant that modulates the activity of the GABA receptors in the brain and nervous system. It promotes relaxation, sleepiness and sometimes a feeling of euphoria. It causes a drop in blood pressure and slows the pulse rate. These properties are the reason why it was initially thought to be a useful sedative and anxiolytic. Common side effects of Methaqualone include dizziness, nausea, vomiting, diarrhea, abdominal cramps, fatigue, itching, rashes, sweating, dry mouth, tingling sensation in arms and legs, seizures and its depressant effects include reduced heart rate and respiration. The drug became banned in many countries and was withdrawn from many markets in the early 1980s.

Fenethylline (generic name Captagon) is a codrug of amphetamine and theophylline. In the fenetylline molecule, theophylline is covalently linked with amphetamine via an alkyl chain. It was formerly used to treat conditions such as ADHD, narcolepsy, and depression, but its use has been banned because of the potential for abuse. Amphetamine, an agonist for trace amine-associated receptor 1 (TAAR1) with enhancing dopamine signaling (an increase of irritability, aggression, etc.), is the main cause of Captagon addiction. Theophylline, an antagonist that blocks adenosine receptors (e.g. A2aR) in the brain responsible for restlessness and painlessness, may attenuate the behavioral sensitization caused by amphetamine. Fenethylline is included in a list of compounds to be considered by a World Health Organization (WHO) Expert Committee in April 1985 for possible international scheduling under the Convention on Psychotropic Substances, 1971. Fenethylline re-emerged because of its widespread abuse by Middle Eastern young adults. Terrorist groups such as the Islamic State to enhance what they consider desirable characteristics - aggressiveness, alertness, and fearlessness - in their recruits, promote it.