Cholecystokinin (CCK) is a peptide hormone produced in the small intestine by enteroendocrine cells in the duodenum. It is a member of the gastrin/cholecystokinin family of peptide hormones and is very similar in structure to gastrin, another gastrointestinal hormone. CCK plays a fundamental role in the digestive process by stimulating the digestion of fat and protein. It is responsible for regulating the release of digestive enzymes and bile from the pancreas and gallbladder, respectively. CCK receptors are found in the muscles of the gallbladder, the mucosal lining of the stomach and intestines, and the lining of the pancreas. CCK also reduces appetite by signaling satiety to the brain.

CCK exists in several forms, each identified by the number of amino acids it contains, e.g., CCK-58, CCK-33, CCK-22, and CCK-8. Most CCK peptides have a sulfate group attached to a tyrosine located seven residues from the C-terminus, which is crucial for the ability of CCK to activate the cholecystokinin A receptor. CCK has been shown to interact with the Cholecystokinin A receptor located mainly on pancreatic acinar cells and the Cholecystokinin B receptor mostly in the brain and stomach.

CCK is not directly involved in any medical treatment to date, but it has been used in drug development and testing. For example, researchers are attempting to create hunger-suppressing drugs that mimic the effects of CCK. A particular fragment of CCK called CCK-4 is known to produce anxiety and panic in humans, and scientists often use it to test new anxiety-reducing drugs.