An antagonist works by preventing the neurotransmitter from properly attaching to its corresponding receptor. For example, many stimulants, which are medications that increase arousal levels, interfere with the proper functioning of the dopamine neurotransmitter system by preventing the neurotransmitter from being reabsorbed into its original neuron (OpenStax, 2016). Caffeine is a specific drug that acts like a stimulant, but binds to a different receptor than most stimulants. Caffeine is an adenosine receptor antagonist (OpenStax, 2016). Although many people do not consider caffeine a drug, some people may become addicted to the drug's ability to increase their energy levels. Similar to caffeine acting on a different neurotransmitter system than most stimulants, hallucinogens, another group of drugs that change a person's mental state, do not have an effect on a specific system. Different hallucinogens affect different neurotransmitter systems and therefore may interrupt multiple systems of the nervous system.