The development of symptoms follows an orderly sequence that varies with the intensity of the stimulus and the susceptibility of the individual. The initial symptom is usually vague discomfort around the upper abdomen ("stomach awareness"), which is followed by nausea and increasing malaise. The face or area around the mouth becomes pale and general sweating begins. With rapid worsening of symptoms ("avalanche syndrome") there can be increased salivation, feelings of body warmth, lightheadedness and often depression and apathy. Vomiting typically follows.
Additional symptoms are frequent, but more variable. These include belching and flatulence, hyperventilation, sighing and yawning, headache, tightness around the forehead or a "buzzing" sensation, drowsiness, lethargy and somnolence, panic or confusion. The lethargy, fatigue, and drowsiness can persist after the stimulus stops and nausea lessens.
Over time, there is a tendency to adapt ("to get one's sea legs"). For most individuals this occurs by 2 to 3 days, although about 5% are said not to adapt and remain symptomatic if the stimulus persists. Returning to stable circumstances, as in returning to shore, can trigger an exacerbation, but this is usually shorter because readaptation is quicker. Personal susceptibility tends to be a stable and enduring characteristic, and is predictive of greater susceptibility in the future.
Important physical characteristics of the stimulus include the frequency, intensity, and duration of directional changes. It is increased by visual stimuli, such as a moving horizon, or by zero gravity. Rates are magnified by other environmental factors, such as poor ventilation, odors, fumes, smoke, and carbon monoxide.
Some people are more prone to this condition than others, but factors such as turbulence, anxiety and illness can also trigger motion sickness. As an indication, the market for travel sickness remedies in the UK in 1986 was £3.3 million.