In a medical or biological context stress is a physical, mental, or emotional factor that causes bodily or mental tension. Stresses can be external (from the environment, psychological, or social situations) or internal (illness, or from a medical procedure). Stress can initiate the "fight or flight" response, a complex reaction of neurologic and endocrinologic systems.

Catecholamine hormones, such as adrenaline or noradrenaline, facilitate immediate physical reactions associated with a preparation for violent muscular action. These include the following: Acceleration of heart and lung action, paling or flushing, or alternating between both, inhibition of stomach and upper-intestinal action to the point where digestion slows down or stops, the general effect on the sphincters of the body, constriction of blood vessels in many parts of the body, liberation of nutrients (particularly fat and glucose) for muscular action, dilation of blood vessels for muscles, inhibition of the lacrimal gland (responsible for tear production) and salivation, dilation of pupil (mydriasis), relaxation of bladder, inhibition of erection, auditory exclusion (loss of hearing), tunnel vision (loss of peripheral vision), disinhibition of spinal reflexes, and Shaking

Stress can cause or influence the course of many medical conditions including psychological conditions such as depression and anxiety. Medical problems can include poor healing, irritable bowel syndrome, high blood pressure, poorly controlled diabetes and many other conditions. Stress management is recognized as an effective treatment modality to include pharmacologic and non-pharmacologic components.