Food intolerance are all other adverse reactions to food. Subgroups include enzymatic (e.g. lactose intolerance due to lactase deficiency), pharmacological (e.g. reactions against biogenic amines, histamine intolerance), and undefined food intolerance (e.g. against some food additives).

Food intolerances can be caused by enzymatic defects in the digestive system, can also result from pharmacological effects of vasoactive amines present in foods (e.g. Histamine), among other metabolic, pharmacological and digestive abnormalities.

Allergies and intolerances to a food group may coexist with separate pathologies; for example, cow's milk allergy (CMA) and lactose intolerance are two distinct pathologies.

The prognosis of children diagnosed with intolerance to milk is good: patients respond to diet which excludes cow's milk protein and the majority of patients succeed in forming tolerance. Children with non-IgE-mediated cows milk intolerance have a good prognosis, whereas children with IgE-mediated cows milk allergy in early childhood have a significantly increased risk for persistent allergy, development of other food allergies, asthma and rhinoconjunctivitis.

A study has demonstrated that identifying and appropriately addressing food sensitivity in IBS patients not previously responding to standard therapy results in a sustained clinical improvement and increased overall well being and quality of life.

Milk allergy typically presents in the first year of life. The majority of children outgrow milk allergy by the age of ten years. One large clinical trial reported resolutions of 19% by age 4 years, 42% by age 8 years, 64% by age 12 years, and 79% by 16 years. Children are be able to tolerate milk as an ingredient in baked goods relative to liquid milk. Resolution was more likely if baseline serum IgE was lower, or if IgE-mediated allergy was absent so that all that was present was cell-mediated, non-IgE allergy.

People with confirmed cow's milk allergy may also demonstrate an allergic response to beef, moreso to rare beef versus well-cooked beef. The offending protein appears to be bovine serum albumin. This is not the same beef allergy that is seen primarily in the southeastern United States, triggered by being bitten by a Lone Star tick.

Milk allergy has consequences. In a U.S. government diet and health surveys conducted in 2007-2010, 6,189 children ages 2-17 years were assessed. For those classified as cow's milk allergic at the time of the survey, mean weight, height and body-mass index were significantly lower than their non-allergic peers. This was not true for children with other food allergies. Diet assessment showed a significant 23% reduction of calcium intake and near-significant trends for lower vitamin D and total calorie intake.

The majority of children outgrow egg allergy. One review reported that 70% of children will outgrow this allergy by 16 years. In subsequently published longitudinal studies, one reported that for 140 infants who had challenge-confirmed egg allergy, 44% had resolved by two years. A second reported that for 203 infants with confirmed IgE-mediated egg allergy, 45% resolved by two years of age, 66% by four years, and 71% by six years. Children will be able to tolerate eggs as an ingredient in baked goods and well-cooked eggs sooner than under-cooked eggs. Resolution was more likely if baseline serum IgE was lower, and if the baseline symptoms did not include anaphylaxis.

Incidence and prevalence are terms commonly used in describing disease epidemiology. Incidence is newly diagnosed cases, which can be expressed as new cases per year per million people. Prevalence is the number of cases alive, expressible as existing cases per million people during a period of time. Milk allergies are usually observed in infants and young children, and often disappear with age (see Prognosis), so prevalence of egg allergy may be expressed as a percentage of children under a set age. Milk allergy affects between 2% and 3% of infants in developed countries. This estimate is for antibody-based allergy; prevalence of allergy based on cellular immunity is unknown.

For all age groups, a review of fifty studies conducted in Europe estimated 6.0% for self-reported milk allergy and 0.6% for confirmed. National survey data in the United States collected 2005-2006 showed that from age six and older, the prevalence of serum IgE confirmed milk allergy was under 0.4%.

The most common food allergens account for about 90% of all allergic reactions; in adults they include shellfish, peanuts, tree nuts, fish, and egg. In children, they include milk, eggs, peanuts, and tree nuts. Six to 8% of children under the age of three have food allergies and nearly 4% of adults have food allergies.

For reasons not entirely understood, the diagnosis of food allergies has apparently become more common in Western nations recently. In the United States, food allergy affects as many as 5% of infants less than three years of age and 3% to 4% of adults. A similar prevalence is found in Canada.

About 75% of children who have allergies to milk protein are able to tolerate baked-in milk products, i.e., muffins, cookies, cake, and hydrolyzed formulas.

About 50% of children with allergies to milk, egg, soy, peanuts, tree nuts, and wheat will outgrow their allergy by the age of 6. Those who are still allergic by the age of 12 or so have less than an 8% chance of outgrowing the allergy.

Peanut and tree nut allergies are less likely to be outgrown, although evidence now shows that about 20% of those with peanut allergies and 9% of those with tree nut allergies will outgrow them.

In Japan, allergy to buckwheat flour, used for soba noodles, is more common than peanuts, tree nuts or foods made from soy beans.

Corn allergy may also be prevalent in many populations, although it may be difficult to recognize in areas such as the United States and Canada where corn derivatives are common in the food supply.

Allergies to a specific pollen are usually associated with OAS reactions to other certain foods. For instance, an allergy to ragweed is associated with OAS reactions to banana, watermelon, cantaloupe, honeydew, zucchini, and cucumber. This does not mean that all sufferers of an allergy to ragweed will experience adverse effects from all or even any of these foods. Reactions may be associated with one type of food, with new reactions to other foods developing later. However, reaction to one or more foods in any given category does not necessarily mean a person is allergic to all foods in that group.

An Institute of Medicine report says that food proteins contained in vaccines, such as gelatin, milk, or egg can cause sensitization (development of allergy) in vaccine recipients, to those food items.

OAS produces symptoms when an affected person eats certain fruits, vegetables, and nuts. Some individuals may only show allergy to only one particular food, and others may show an allergic response to many foods.

Individuals with an allergy to tree pollen may develop OAS to a variety of foods. While the tree pollen allergy has been worked out, the grass pollen is not well understood. Furthermore, some individuals have severe reactions to certain fruits and vegetables that do not fall into any particular allergy category. In recent years, it has also become apparent that when tropical foods initiate OAS, allergy to latex may be the underlying cause.

Because the allergenic proteins associated with OAS are usually destroyed by cooking, most reactions are caused by eating raw foods. The main exceptions to this are celery and nuts, which may cause reactions even after being cooked.

When eggs are introduced to a baby's diet is thought to affect risk of developing allergy, but there are contradictory recommendations. A 2016 review acknowledged that introducing peanuts early appears to have a benefit, but stated "The effect of early introduction of egg on egg allergy are controversial." A meta-analysis published the same year supported the theory that early introduction of eggs into an infant's diet lowers risk, and a review of allergens in general stated that introducing solid foods at 4–6 months may result in the lowest subsequent allergy risk. However, an older consensus document from the American College of Allergy, Asthma and Immunology recommended that introduction of chicken eggs be delayed to 24 months of age.

For those allergic to fruits, cooking may help reduce or eliminate the reaction to some fruits.

People with this allergy might not necessarily be allergic to citrus fruits.

Antibodies to α-gliadin have been significantly increased in non-celiacs individuals with oral ulceration. Anti-α-gliadin antibodies are frequently found in celiac disease (CD), to a lesser degree CD, but are also found in a subset who do not have the disease. Of people with pseudo-exfoliation syndrome, 25% showed increased levels of anti-gliadin IgA. Other patients that are also at risk are those taking gluten despite having the disorder, or whose family members with CD. In addition patients with autoimmune conditions are also at risk for CD. It has just been found that there is a risk of death in CD. Therefore gluten intake should be limited before or even after the diagnosis. One fourth of people with Sjögren's syndrome had responses to gluten, of 5 that had positive response to gluten, only one could be confirmed as CD and another was potentially , the remaining 3 appear to be gluten-sensitive. All were HLA-DQ2 and/or DQ8-positive.

There are many different types of fruits that people have been shown to react allergically such as mangoes and bananas. Some foods are clearly more allergenic than others. In adults, peanuts, tree nuts, finned fish, crustaceans, fruit, and vegetables account for 85% of the food-allergic reactions(O'Neil, Zanovec and Nickla).

People suffering from allergies may suffer from a hypersensitivity to the allergic food, which is what causes the allergic reaction. Most fruit allergies are oral syndrome allergies because they are consumed but may also be an external allergy if the fruit touches the skin.

When enteropathy develops in early childhood, symptomatic disease is more rapidly evident. A survey of geriatrics with celiac disease in Finland revealed that the incidence of disease was much higher than the general population. Allergic disease may rise or fall with age; certain evidence points to the increased or daily use of non-steroidal anti-inflammatory factors (aspirin, ibuprofen) as an increased risk factor for urticaria or anaphylaxis, and the sensitizing dose may include low-dose aspirin therapy used in the treatment of heart disease. NCGS may be a late-onset condition: in a prospective study performed among adults of 18 to 80 years, the median age of disease onset was found to be 55 years, with a six times higher prevalence in females than in males.

In a recent double-blind, placebo-controlled challenge (DBPC) by Biesiekierski "et al." in a few patients with IBS, the authors found no difference between gluten or placebo groups and the concept of NCGS as a syndrome was questioned. Nevertheless, probably the reintroduction of both gluten and whey protein had a nocebo effect similar in all patients, and this could have masked the true effect of gluten/wheat reintroduction.

FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides and polyols) that are present in gluten-containing grains have recently been identified as a possible cause of gastrointestinal symptoms in people with NCGS, in place of, or in addition to, gluten. FODMAPs cause mild wheat intolerance mainly limited to gastrointestinal symptoms.

An important salicylate drug is aspirin, which has a long history. Aspirin intolerance was widely known by 1975, when the understanding began to emerge that it is a pharmacological reaction, not an allergy.

The principal symptom of lactose intolerance is an adverse reaction to products containing lactose (primarily milk), including abdominal bloating and cramps, flatulence, diarrhea, nausea, borborygmi, and vomiting (particularly in adolescents). These appear one-half to two hours after consumption. The severity of symptoms typically increases with the amount of lactose consumed; most lactose-intolerant people can tolerate a certain level of lactose in their diets without ill effects.

Lactose intolerance is a condition in which people have symptoms due to the decreased ability to digest lactose, a sugar found in milk products. Those affected vary in the amount of lactose they can tolerate before symptoms develop. Symptoms may include abdominal pain, bloating, diarrhea, gas, and nausea. These symptoms typically start between half and two hours after drinking milk or eating milk products. Severity depends on the amount a person eats or drinks. It does not cause damage to the gastrointestinal tract.

Lactose intolerance is due to the lack of enzyme lactase in the small intestines to break lactose down into glucose and galactose. There are four types: primary, secondary, developmental, and congenital. Primary lactose intolerance is when the amount of lactase declines as people age. Secondary lactose intolerance is due to injury to the small intestine such as from infection, celiac disease, inflammatory bowel disease, or other diseases. Developmental lactose intolerance may occur in premature babies and usually improves over a short period of time. Congenital lactose intolerance is an extremely rare genetic disorder in which little or no lactase is made from birth.

Diagnosis may be confirmed if symptoms resolve following eliminating lactose from the diet. Other supporting tests include a hydrogen breath test and a stool acidity test. Other conditions that may produce similar symptoms include irritable bowel syndrome, celiac disease, and inflammatory bowel disease. Lactose intolerance is different from a milk allergy. Management is typically by decreasing the amount of lactose in the diet, taking lactase supplements, or treating the underlying disease. People are usually able to drink at least one cup of milk per sitting without developing significant symptoms, with greater amounts tolerated if drunk with a meal or throughout the day.

The exact number of adults with lactose intolerance is unknown. One estimate puts the average at 65% of the global population. Rates of lactose intolerance vary between regions, from less than 10% in Northern Europe to as high as 95% in parts of Asia and Africa. Onset is typically in late childhood or early adulthood. The ability to digest lactose into adulthood evolved in several human populations independently probably as an adaptation to domestication of dairy animals 10,000 years ago.

The pathogenesis of NCGS is not yet well understood. It was hypothesized that gluten, as occurs in celiac disease, is the cause of NCGS. Besides gluten, other components in wheat, rye, barley, and their derivatives, including amylasetrypsin inhibitors (ATIs) and FODMAPs, may cause symptoms.

Depending on whether the salicylate is a component of food or medicine, salicylate intolerance is a form of food intolerance or of drug intolerance.

Salicylate sensitivity is a pharmacological reaction, not a true IgE-mediated allergy. However, it is possible for aspirin to trigger non-allergic hypersensitivity reactions. About 5–10% of asthmatics have aspirin hypersensitivity, but dietary salicylates have been shown not to contribute to this. The reactions in AERD (Samter's triad) are due to inhibition of the COX-1 enzyme by aspirin, as well as other NSAIDs that are not salicylates. Dietary salicylates have not been shown to significantly affect COX-1.

Samter's triad refers to aspirin sensitivity in conjunction with nasal polyps and asthma.

Other cereals such as corn, millet, sorghum, teff, rice, and wild rice are safe for people with coeliac to consume, as well as noncereals such as amaranth, quinoa, and buckwheat. Noncereal carbohydrate-rich foods such as potatoes and bananas do not contain gluten and do not trigger symptoms.

There are various theories as to what determines whether a genetically susceptible individual will go on to develop coeliac disease. Major theories include surgery, pregnancy, infection and emotional stress.

The eating of gluten early in a baby's life does not appear to increase the risk of CD but later introduction after 6 months may increase it. There is uncertainty whether breastfeeding reduces risk. Prolonging breastfeeding until the introduction of gluten-containing grains into the diet appears to be associated with a 50% reduced risk of developing coeliac disease in infancy; whether this persists into adulthood is not clear. These factors may just influence the timing of onset.

Risk factors for drug allergies can be attributed to the drug itself or the characteristics of the patient. Drug-specific risk factors include the dose, route of administration, duration of treatment, repetitive exposure to the drug, and concurrent illnesses. Host risk factors include age, sex, atopy, specific genetic polymorphisms, and inherent predisposition to react to multiple unrelated drugs (multiple drug allergy syndrome).

A drug allergy is more likely to develop with large doses and extended exposure.

When a medication causes an allergic reaction, it is called an allergen. The following is a short list of the most common drug allergens:

- Antibiotics

- Penicillin

- Sulfa drugs

- Tetracycline

- Analgesics

- Codeine

- Non-steroidal anti-inflammatory drugs (NSAIDs)

- Antiseizure

- Phenytoin

- Carbamazepine

GSE can result in high risk pregnancies and infertility. Some infertile women have GSE and iron deficiency anemia others have zinc deficiency and birth defects may be attributed to folic acid deficiencies.

It has also been found to be a rare cause of amenorrhea.