The skin is another source of potential sensitivity, although the thickness of the skin layer usually prevents severe reactions in most people. In general, orally ingested chemicals or inhalants cause chemical sensitivity. Although the mechanism of chemical sensitivity is similar whether the stimulus is inhaled or ingested, I will focus on the effect of orally ingested stimuli on the mucosa of the GI tract. Various researchers have hypothesized that leaky gut syndrome is the real culprit in chemical sensitivity to foods and medications. According to this theory, the mucosa becomes leaky because the junction between cells is somewhat open.
Most explanations blame the exogenous food substance for the changed response.
I believe that the mechanism of chemical sensitivity works somewhat differently for patients with fibromyalgia. Or, the patient may tolerate 10 mg of a medication, but 20 mg causes a deleterious response. Some patients say if they eat one egg or other food, they have no problem, but a second or third serving causes a reaction. Other patients explain that a sudden sensitivity to a particular food they have eaten for years has caused them to eliminate the food from their diet. If the exogenous compound is the problem, the first tablet, egg, or other food should cause the reaction, not the second, third, fourth, or 2000th.