Worsening or precipitation of symptoms may result from avoidance of the culprit food - a kind of withdrawal effect. Thus, a person who suffers with migraine due to food intolerance may find that missing breakfast triggers a migraine attack. Very careful elimination and challenge testing of that patient's regular breakfast foods then usually identifies the culprit food, for example milk, wheat or corn. The existence of a withdrawal response can also explain why the food intolerant patient is often mildly addicted (often without realizing it) to the food or foods causing their problem. The child with catarrh, recurrent ear infection, insomnia and irritability who craves regular milk, cheese and yogurt, is a good example. Night-time waking, only settling after a drink of milk, is an even stronger clue. The complete disappearance of symptoms with milk avoidance (and recurrence with milk reintroduction) is then clear evidence of the relationship.
This is the basis for the diagnostic test that is necessary to confirm the existence of food intolerance. Ten days of scrupulous avoidance effectively unmasks the hidden intolerance, so that eating the food for the first time after the period of avoidance usually produces symptoms within an hour or two. The avoidance phase is called an elimination diet and during this phase, the symptoms should get better (although there may be an initial 'withdrawal' worsening, as explained above); the reintroduction phase is called the food challenge. Attention to detail is needed during the elimination phase. For example, if sensitivity to milk is the cause of symptoms, they may not disappear unless all forms of milk are excluded (milk, butter, yoghurt, cheese and milk products such as lactose, casein and whey in cakes, biscuits and other processed foods). This may require, for example, the avoidance of toothpaste and certain medications.
Weeks or months of elimination of the reactive food may well lead to re-introduction of the food without reaction. This is known as tolerance, and its maintenance depends on establishing the threshold of both frequency and quantity for that person - in other words, eating the food occasionally may be tolerated, but re-introducing it in large quantities or on a very regular basis (e.g. every day) might lead to symptoms recurring.