Caffeine is a methylxanthine alkaloid found in the seeds, nuts, or leaves of a number of plants native to South America and East Asia that is structurally related to adenosine and acts primarily as an adenosine receptor antagonist with psychotropic and anti-inflammatory activities. Upon ingestion, caffeine binds to adenosine receptors in the central nervous system (CNS), which inhibits adenosine binding. This inhibits the adenosine-mediated downregulation of CNS activity; thus, stimulating the activity of the medullary, vagal, vasomotor, and respiratory centers in the brain. The anti-inflammatory effects of caffeine are due the nonselective competitive inhibition of phosphodiesterases. Caffeine is used by mouth or rectally in combination with painkillers (such as aspirin and acetaminophen) and a chemical called ergotamine for treating migraineheadaches. It is also used with painkillers for simple headaches and preventing and treating headaches after epidural anesthesia. Caffeine creams are applied to the skin to reduce redness and itching in dermatitis. Healthcare providers sometimes give caffeine intravenously (by IV) for headache after epidural anesthesia, breathing problems in newborns, and to increase urine flow. In foods, caffeine is used as an ingredient in soft drinks, energy drinks, and other beverages.

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.