An antagonist works by preventing a neurotransmitter from attaching properly to its corresponding receptor. For example, many stimulants, which are drugs that increase arousal levels, interfere with the proper functioning of the dopamine neurotransmitter system by blocking the reabsorption of the neurotransmitter into its original neuron (OpenStax, 2016). One specific drug that acts as a stimulant, but binds to a different receptor than most stimulants, is caffeine. Caffeine is an adenosine receptor antagonist (OpenStax, 2016). Although many people do not consider caffeine to be a drug, individuals can become addicted and dependent on the drug's ability to increase energy levels. Similar to caffeine acting on a different neurotransmitter system than most stimulants, hallucinogens, another group of drugs that alter a person's mental state, do not have a specific system they act on. Different hallucinogens affect different neurotransmitter systems and therefore can disrupt different nervous systems