Any factor or event(s) that threaten(s) a person's health or adversely affect(s) his or her normal functioning. Injury, disease or worry are common examples; others include internal conflicts, emotive life events – such as the death of a close relative or friend, the birth of a baby, separation or divorce – pressures at work or a hostile environment such as war or famine. Some individuals seem to be more prone than others to develop medical problems related to stress.

Stress prompts the body to raise its output of HORMONES such as ADRENALINE and CORTISOL, causing changes in blood pressure, heart rate and metabolism. These are physiological responses intended to improve a person's physical and mental performance – the ‘fight or flight’ reaction to fear. Stress may, however, disrupt the ability to cope. Constant or recurrent exposure to stress may produce symptoms such as anxiety, depression, headaches, indigestion, diarrhoea, palpitations and general malaise (see POST-TRAUMATIC STRESS DISORDER (PTSD)). Treatment can be difficult and prolonged; counselling can help as can ANXIOLYTICS or ANTIDEPRESSANT DRUGS – but a change in job or lifestyle may be necessary in some circumstances.