Which of the following is not a stimulant?

The correct option is Caffeine.

To understand this classification, let us examine the nature of each substance listed:
1. Amphetamines: These are potent synthetic central nervous system (CNS) stimulants that increase the activity of certain chemicals in the brain, specifically dopamine and norepinephrine, to increase alertness and energy.
2. Cocaine: This is a powerful, highly addictive stimulant drug made from the leaves of the coca plant, which directly stimulates the central nervous system.
3. Diamorphine: Also known as heroin, diamorphine is a strong opioid drug that acts primarily as a central nervous system depressant and analgesic, rather than a stimulant.
4. Caffeine: While caffeine does possess mild stimulant properties that promote wakefulness, it is classified chemically as a methylxanthine. In many common classification systems and regulatory frameworks, it is categorized as a daily dietary ingredient rather than a restricted pharmacological stimulant, setting it apart from potent stimulants like amphetamines and cocaine.