Identifying a drug allergy can sometimes be the hardest part. Sometimes drug allergies are confused with nonallergic drug reactions because they both cause somewhat similar reactions. Symptoms of a drug allergy can include, but are not limited to, the following list.

- Hives

- Itching

- Rash

- Fever

- Facial swelling

- Shortness of breath due to the short-term constriction of lung airways or longer-term damage to lung tissue

- Anaphylaxis, a life-threatening drug reaction (produces most of these symptoms as well as low blood pressure)

- Cardiac symptoms such as chest pain, shortness of breath, fatigue, chest palpitations, light headedness, and syncope due to a rare drug-induced reaction, eosinophilic myocarditis

Treatment usually involves adrenaline (epinephrine), antihistamines, and corticosteroids.

If the entire body is involved, then anaphylaxis can take place, which is an acute, systemic reaction that can prove fatal.

Many allergens such as dust or pollen are airborne particles. In these cases, symptoms arise in areas in contact with air, such as eyes, nose, and lungs. For instance, allergic rhinitis, also known as hay fever, causes irritation of the nose, sneezing, itching, and redness of the eyes. Inhaled allergens can also lead to increased production of mucus in the lungs, shortness of breath, coughing, and wheezing.

Aside from these ambient allergens, allergic reactions can result from foods, insect stings, and reactions to medications like aspirin and antibiotics such as penicillin. Symptoms of food allergy include abdominal pain, bloating, vomiting, diarrhea, itchy skin, and swelling of the skin during hives. Food allergies rarely cause respiratory (asthmatic) reactions, or rhinitis. Insect stings, food, antibiotics, and certain medicines may produce a systemic allergic response that is also called anaphylaxis; multiple organ systems can be affected, including the digestive system, the respiratory system, and the circulatory system. Depending on the rate of severity, it can cause a skin reactions, bronchoconstriction, swelling, low blood pressure, coma, and death. This type of reaction can be triggered suddenly, or the onset can be delayed. The nature of anaphylaxis is such that the reaction can seem to be subsiding, but may recur throughout a period of time.

NSAID or nonsteroidal anti-inflammatory drug hypersensitivity reactions encompasses a broad range of allergic or allergic-like symptoms that occur within minutes to hours after ingesting aspirin or other NSAID nonsteroidal anti-inflammatory drugs. Hypersensitivity drug reactions differ from drug toxicity reactions in that drug toxicity reactions result from the pharmacological action of a drug, are dose-related, and can occur in any treated individual (see nonsteroidal anti-inflammatory drugs section on adverse reactions for NSAID-induced toxic reactions); hypersensitivity reactions are idiosyncratic reactions to a drug. Although the term NSAID was introduced to signal a comparatively low risk of adverse effects, NSAIDs do evoke a broad range of hypersensitivity syndromes. These syndromes have recently been classified by the European Academy of Allergy and Clinical Immunology Task Force on NSAIDs Hypersensitivity. The classification organizes the hypersensitivity reactions to NSAIDs into the following five categories:

- 1) NSAIDs-exacerbated respiratory disease (NERD) is an acute (immediate to several hours) exacerbation of bronchoconstriction and other symptoms of asthma (see aspirin-induced asthma) in individuals with a history of asthma and/or nasal congestion, rhinorrhea or other symptoms of rhinitis and sinusitis in individuals with a history of rhinosinusitis after ingestion of various NSAIDs, particularly those that act by inhibiting the COX-1 enzyme. NERD does not appear to be due to a true allergic reaction to NSAIDs but rather at least in part to the more direct effects of these drugs to promote the production and/or release of certain mediators of allergy. That is, inhibition of cellular COX activity deprives tissues of its anti-inflammatory product(s), particularly prostaglandin E2 while concurrently shuttling its substrate, arachidonic acid, into other metabolizing enzymes, particularly 5-lipoxygenase (ALOX5) to overproduce pro-inflammatory leukotriene and 5-Hydroxyicosatetraenoic acid metabolites and 15-lipoxygenase (ALOX15) to overproduce pro-inflammatory 15-Hydroxyicosatetraenoic acid metabolites, including eoxins; the condition is also associated with a reduction in the anti-inflammatory metabolite, lipoxin A4, and increases in certain pro-allergic chemokines such as eotaxin-2 and CCL7.

- 2) NSAIDs-exacerbated cutaneous disease (NECD) is an acute exacerbation of wheals and/or angioedema in individuals with a history of chronic urticaria. NECD also appears due to the non-allergic action of NSAIDs in inhibiting the production of COX anti-inflammatory metabolites while promoting the production 5-lipoxygenase and 15-lipoxygenase pro-inflammatory metabolites and the overproduction of certain pro-allergic chemokines, e.g. eotaxin-1, eotaxin-2, RANTES, and interleukin-5.

- 3) NSAIDs-induced urticarial disease (NEUD) is the acute development of wheals and/or angioedema in individuals with no history of chronic NSAIDs-induced urticaria or related diseases. The mechanism behind NEUD is unknown but may be due to the non-allergic action of NSAIDs in promoting the production and/or release of allergy mediators.

- 4) Single NSAID-induced urticarial/angioedema or anaphylaxis (SNIUAA) is the acute development of urticarial, angioedema, or anaphylaxis in response to a single type of NSAID and/or a single group of NSAIDs with a similar structure but not to other structurally unrelated NSAIDs in individuals with no history of underlying relevant chronic diseases. SNIUAA is due to a true IgE-mediated allergy reaction.

- 5 Single NSAID-induced delayed reactions (SNIDR) are a set of delayed onset (usually more than 24 hour) reactions to NSAIDs. SNIDR are most commonly skin reactions that may be relatively mild moderately severe such as maculopapular rash, fixed drug eruptions, photosensitivity reactions, delayed urticaria, and contact dermatitis or extremely severe such as the DRESS syndrome, acute generalized exanthematous pustulosis, the Stevens–Johnson syndrome, and toxic epidermal necrolysis (also termed Lyell's syndrome). SNIDR result from the drug-specific stimulation of CD4+ T lymphocytes and CD8+ cytotoxic T cells to elicit a delayed type hypersensitivity reaction.

Some examples:

- Allergic asthma

- Allergic conjunctivitis

- Allergic rhinitis ("hay fever")

- Anaphylaxis

- Angioedema

- Urticaria (hives)

- Eosinophilia

- Penicillin allergy

- Cephalosporin allergy

- Food allergy

- Sweet itch

Anaphylaxis typically presents many different symptoms over minutes or hours with an average onset of 5 to 30 minutes if exposure is intravenous and 2 hours if from eating food. The most common areas affected include: skin (80–90%), respiratory (70%), gastrointestinal (30–45%), heart and vasculature (10–45%), and central nervous system (10–15%) with usually two or more being involved.

The various non-allergic NSAID hypersensitivity syndromes affect 0.5–1.9% of the general population, with AERD affecting about 7% of all asthmatics and about 14% of patients with severe asthma. AERD, which is more prevalent in women, usually begins in young adulthood (twenties and thirties are the most common onset times although children are afflicted with it and present a diagnostic problem in pediatrics) and may not include any other allergies. Most commonly the first symptom is rhinitis (inflammation or irritation of the nasal mucosa), which can manifest as sneezing, runny nose, or congestion. The disorder typically progresses to asthma, then nasal polyposis, with aspirin sensitivity coming last. Anosmia (lack of smell) is also common, as inflammation within the nose and sinuses likely reaches the olfactory receptors.

The respiratory reactions to aspirin vary in severity, ranging from mild nasal congestion and eye watering to lower respiratory symptoms including wheezing, coughing, an asthma attack, and in rare cases, anaphylaxis. In addition to the typical respiratory reactions, about 10% of patients with AERD manifest skin symptoms like urticaria and/or gastrointestinal symptoms such as abdominal pain or vomiting during their reactions to aspirin.

In addition to aspirin, patients usually also react to other NSAIDs such as ibuprofen, and to any medication that inhibits the cyclooxygenase-1 (COX-1) enzyme, although paracetamol (acetaminophen) in low doses is generally considered safe. NSAID that are highly selective in blocking COX-2 and do not block its closely related paralog, COX-1, such as the COX-2 inhibitors celecoxib and rofecoxib, are also regarded as safe. Nonetheless, recent studies do find that these types of drugs, e.g. acetaminophen and celecoxib, may trigger adverse reactions in these patients; caution is recommended in using any COX inhibitors. In addition to aspirin and NSAIDs, consumption of even small amounts of alcohol also produces uncomfortable respiratory reactions in many patients.

A drug allergy is an allergy to a drug, most commonly a medication, and is a form of adverse drug reaction. Medical attention should be sought immediately if an allergic reaction is suspected.

An allergic reaction will not occur on the first exposure to a substance. The first exposure allows the body to create antibodies and memory lymphocyte cells for the antigen. However, drugs often contain many different substances, including dyes, which could cause allergic reactions. This can cause an allergic reaction on the first administration of a drug. For example, a person who developed an allergy to a red dye will be allergic to any new drug which contains that red dye.

A drug allergy is different from an intolerance. A drug intolerance, which is often a milder, non-immune-mediated reaction, does not depend on prior exposure. Most people who believe they are allergic to aspirin are actually suffering from a drug intolerance.

Allergic inflammation is an important pathophysiological feature of several disabilities or medical conditions including allergic asthma, atopic dermatitis, allergic rhinitis and several ocular allergic diseases. Allergic reactions may generally be divided into two components; the early phase reaction, and the late phase reaction. While the contribution to the development of symptoms from each of the phases varies greatly between diseases, both are usually present and provide us a framework for understanding allergic disease.

The early phase of the allergic reaction typically occurs within minutes, or even seconds, following allergen exposure and is also commonly referred to as the immediate allergic reaction or as a Type I allergic reaction. The reaction is caused by the release of histamine and mast cell granule proteins by a process called degranulation, as well as the production of leukotrienes, prostaglandins and cytokines, by mast cells following the cross-linking of allergen specific IgE molecules bound to mast cell FcεRI receptors. These mediators affect nerve cells causing itching, smooth muscle cells causing contraction (leading to the airway narrowing seen in allergic asthma), goblet cells causing mucus production, and endothelial cells causing vasodilatation and edema.

The late phase of a Type 1 reaction (which develops 8–12 hours and is mediated by mast cells) should not be confused with delayed hypersensitivity Type IV allergic reaction (which takes 48–72 hours to develop and is mediated by T cells). The products of the early phase reaction include chemokines and molecules that act on endothelial cells and cause them to express Intercellular adhesion molecule (such as vascular cell adhesion molecule and selectins), which together result in the recruitment and activation of leukocytes from the blood into the site of the allergic reaction. Typically, the infiltrating cells observed in allergic reactions contain a high proportion of lymphocytes, and especially, of eosinophils. The recruited eosinophils will degranulate releasing a number of cytotoxic molecules (including Major Basic Protein and eosinophil peroxidase) as well as produce a number of cytokines such as IL-5. The recruited T-cells are typically of the Th2 variety and the cytokines they produce lead to further recruitment of mast cells and eosinophils, and in plasma cell isotype switching to IgE which will bind to the mast cell FcεRI receptors and prime the individual for further allergic responses.

Symptoms typically include generalized hives, itchiness, flushing, or swelling (angioedema) of the afflicted tissues. Those with angioedema may describe a burning sensation of the skin rather than itchiness. Swelling of the tongue or throat occurs in up to about 20% of cases. Other features may include a runny nose and swelling of the conjunctiva. The skin may also be blue tinged because of lack of oxygen.

Atopy (atopic syndrome) is a syndrome characterized by a tendency to be “hyperallergic”. A person with atopy typically presents with one or more of the following: eczema (atopic dermatitis), allergic rhinitis (hay fever), or allergic asthma. Some patients with atopy display what is referred to as the “allergic triad” of symptoms, i.e. all three of the aforementioned conditions. Patients with atopy also have a tendency to have food allergies, allergic conjunctivitis, and other symptoms characterized by their hyperallergic state. For example, eosinophilic esophagitis is found to be associated with atopic allergies.

Atopic syndrome can be fatal for those who experience serious allergic reactions, such as anaphylaxis, brought on by reactions to food or environment.

Allergies, also known as allergic diseases, are a number of conditions caused by hypersensitivity of the immune system to something in the environment that usually causes little or no problem in most people. These diseases include hay fever, food allergies, atopic dermatitis, allergic asthma, and anaphylaxis. Symptoms may include red eyes, an itchy rash, sneezing, a runny nose, shortness of breath, or swelling. Food intolerances and food poisoning are separate conditions.

Common allergens include pollen and certain food. Metals and other substances may also cause problems. Food, insect stings, and medications are common causes of severe reactions. Their development is due to both genetic and environmental factors. The underlying mechanism involves immunoglobulin E antibodies (IgE), part of the body's immune system, binding to an allergen and then to a receptor on mast cells or basophils where it triggers the release of inflammatory chemicals such as histamine. Diagnosis is typically based on a person's medical history. Further testing of the skin or blood may be useful in certain cases. Positive tests, however, may not mean there is a significant allergy to the substance in question.

Early exposure to potential allergens may be protective. Treatments for allergies include avoiding known allergens and the use of medications such as steroids and antihistamines. In severe reactions injectable adrenaline (epinephrine) is recommended. Allergen immunotherapy, which gradually exposes people to larger and larger amounts of allergen, is useful for some types of allergies such as hay fever and reactions to insect bites. Its use in food allergies is unclear.

Allergies are common. In the developed world, about 20% of people are affected by allergic rhinitis, about 6% of people have at least one food allergy, and about 20% have atopic dermatitis at some point in time. Depending on the country about 1–18% of people have asthma. Anaphylaxis occurs in between 0.05–2% of people. Rates of many allergic diseases appear to be increasing. The word "allergy" was first used by Clemens von Pirquet in 1906.

Aspirin-induced asthma, also termed Samter's triad, Samter's syndrome, aspirin-exacerbated respiratory disease (AERD), and recently by an appointed task force of the European Academy of Allergy and Clinical Immunology/World Allergy Organization (EAACI/WAO) Nonsteroidal anti-inflammatory drugs-exacerbated respiratory disease (N-ERD). is a medical condition initially defined as consisting of three key features: asthma, respiratory symptoms exacerbated by aspirin, and nasal/ethmoidal polyposis; however, the syndrome's symptoms are exacerbated by a large variety of other nonsteroidal anti-inflammatory drugs (NSAIDs) besides aspirin. The symptoms of respiratory reactions in this syndrome are hypersensitivity reactions to NSAIDs rather than the typically described true allergic reactions that trigger other common allergen-induced asthma, rhinitis, or hives. The NSAID-induced reactions do not appear to involve the common mediators of true allergic reactions, immunoglobulin E or T cells. Rather, AERD is a type of NSAID-induced hypersensitivity syndrome. EAACI/WHO classifies the syndrome as one of 5 types of NSAID hypersensitivity or NSAID hypersensitivity reactions.

The symptoms may vary depending upon the person, the severity of the allergy, and type of fruit. For example, mango allergy symptoms include hoarseness, dyspnoea and bronchitic rales (asthma) (Sareen and Shah). The duration of the symptoms tested by Saree and Shah were variable and ranged from 4 h [11] to 7 days [12]. The symptoms may appear within a few minutes.

Atopy is a predisposition toward developing certain allergic hypersensitivity reactions.

Atopy may have a hereditary component, although contact with the allergen or irritant must occur before the hypersensitivity reaction can develop. Maternal psychological trauma in utero may also be a strong indicator for development of atopy.

The term "atopy" was coined by Coca and Cooke in 1923. Many physicians and scientists use the term "atopy" for any IgE-mediated reaction (even those that are appropriate and proportional to the antigen), but many pediatricians reserve the word "atopy" for a genetically mediated predisposition to an excessive IgE reaction. The term is from Greek ἀτοπία meaning "placelessness".

Type IV allergy, also known as allergic contact dermatitis, involves a delayed skin rash that is similar to poison ivy with blistering and oozing of the skin ("see urushiol-induced contact dermatitis"). It can be diagnosed through a positive skin patch test, although a negative test does not rule out a latex allergy.

Severe irritation takes place if a latex catheter is inserted in the urinary tract of a person allergic to latex. That is especially severe in case of a radical prostatectomy due to the open wound there and the exposure lasting e.g. two weeks. Intense pain may indicate such situation.

Allergic reactions to fruit and vegetables are usually mild and often just affect the mouth, causing itching, a rash, or blisters where the food touches the lips and mouth. This is called oral allergy syndrome. A number of people who react in this way to fruit or vegetables will also react to pollen from some trees and weeds. So, for example, people who are allergic to birch pollen are also likely to be allergic to apples.

Another symptom may include slight swelling in the throat, making it feel like it is closing. The ability to breathe is still present though, so it is not fatal.

Natural rubber latex can also cause irritant contact dermatitis, a less severe form of reaction that does not involve the immune system. Contact dermatitis causes dry, itchy, irritated areas on the skin, most often on the hands. Latex-glove induced dermatitis increases the chance of hospital-acquired infections, including blood-borne infections, being transmitted.

Food allergies usually have a fast onset (from seconds to one hour) and may include:

- Rash

- Hives

- Itching of mouth, lips, tongue, throat, eyes, skin, or other areas

- Swelling (angioedema) of lips, tongue, eyelids, or the whole face

- Difficulty swallowing

- Runny or congested nose

- Hoarse voice

- Wheezing and/or shortness of breath

- Diarrhea, abdominal pain, and/or stomach cramps

- Lightheadedness

- Fainting

- Nausea

- Vomiting

In some cases, however, onset of symptoms may be delayed for hours.

Symptoms of allergies vary from person to person. The amount of food needed to trigger a reaction also varies from person to person.

Serious danger regarding allergies can begin when the respiratory tract or blood circulation is affected. The former can be indicated through wheezing and cyanosis. Poor blood circulation leads to a weak pulse, pale skin and fainting.

A severe case of an allergic reaction, caused by symptoms affecting the respiratory tract and blood circulation, is called anaphylaxis. When symptoms are related to a drop in blood pressure, the person is said to be in anaphylactic shock. Anaphylaxis occurs when IgE antibodies are involved, and areas of the body that are not in direct contact with the food become affected and show symptoms. Those with asthma or an allergy to peanuts, tree nuts, or seafood are at greater risk for anaphylaxis.

Peanut allergy is a type of food allergy to peanuts. It is different from nut allergies. Physical symptoms of allergic reaction can include itchiness, urticaria, swelling, eczema, sneezing, asthma, abdominal pain, drop in blood pressure, diarrhea, and cardiac arrest. Anaphylaxis may occur.

It is due to a type I hypersensitivity reaction of the immune system in susceptible individuals. The allergy is recognized "as one of the most severe food allergies due to its prevalence, persistency, and potential severity of allergic reaction."

Prevention may be partly achieved through early introduction of peanuts to the diets of pregnant women and babies. The principal treatment for anaphylaxis is epinephrine as an injection.

In the United States, peanut allergy is present in 0.6% of the population. It is a common cause of food-related fatal and near-fatal allergic reactions.

Food allergies usually have a fast onset (from seconds to one hour). Symptoms may include: rash, hives, itching of mouth, lips, tongue, throat, eyes, skin, or other areas, swelling of lips, tongue, eyelids, or the whole face, difficulty swallowing, runny or congested nose, hoarse voice, wheezing, shortness of breath, diarrhea, abdominal pain, lightheadedness, fainting, nausea, or vomiting. Symptoms of allergies vary from person to person and may vary from incident to incident. Serious danger regarding allergies can begin when the respiratory tract or blood circulation is affected. The former can be indicated by wheezing, a blocked airway and cyanosis, the latter by weak pulse, pale skin, and fainting. When these symptoms occur the allergic reaction is called anaphylaxis. Anaphylaxis occurs when IgE antibodies are involved, and areas of the body that are not in direct contact with the food become affected and show severe symptoms. Untreated, this can proceed to vasodilation, a low blood pressure situation called anaphylactic shock, and death (very rare).

Young children may exhibit dermatitis/eczema on face, scalp and other parts of the body, in older children knees and elbows are more commonly afflicted. Children with dermatitis are at greater than expected risk of also exhibiting asthma and allergic rhinitis.

The symptoms of allergic contact dermatitis are very similar to the ones caused by irritant contact dermatitis, which makes the first even harder to diagnose. The first sign of allergic contact dermatitis is the presence of the rash or skin lesion at the site of exposure. Depending on the type of allergen causing it, the rash can ooze, drain or crust and it can become raw, scaled or thickened. Also, it is possible that the skin lesion does not take the form of a rash but it may include papules, blisters, vesicles or even a simple red area. The main difference between the rash caused by allergic contact dermatitis and the one caused by irritant contact dermatitis is that the latter tends to be confined to the area where the trigger touched the skin, whereas in allergic contact dermatitis the rash is more likely to be more widespread on the skin. Another characteristic of the allergic contact dermatitis rash is that it usually appears after a day or two after exposure to the allergen, unlike irritant contact dermatitis that appears immediately after the contact with the trigger.

Other symptoms may include itching, skin redness or inflammation, localized swelling and the area may become more tender or warmer. If left untreated, the skin may darken and become leathery and cracked. Pain can also be present.

The symptoms of allergic contact may persist for as long as one month before resolving completely. Once an individual has developed a skin reaction to a certain substance it is most likely that they will have it for the rest of their life, and the symptoms will reappear when in contact with the allergen.

Food allergies can have fast onset (from seconds to one hour) or slow onset (from hours to several days) depending on mechanism. Symptoms may include: rash, hives, itching of mouth, lips, tongue, throat, eyes, skin, or other areas, swelling of lips, tongue, eyelids, or the whole face, difficulty swallowing, runny or congested nose, hoarse voice, wheezing, shortness of breath, diarrhea, abdominal pain, lightheadedness, fainting, nausea and vomiting. Symptoms of allergies vary from person to person and may vary from incident to incident. Serious danger regarding allergies can begin when the respiratory tract or blood circulation is affected. The former can be indicated by wheezing, a blocked airway and cyanosis, the latter by weak pulse, pale skin, and fainting. When these symptoms occur the allergic reaction is called anaphylaxis. Anaphylaxis occurs when IgE antibodies are involved, and areas of the body that are not in direct contact with the food become affected and show severe symptoms. Untreated, this can proceed to vasodilation, a low blood pressure situation called anaphylactic shock, and death (very rare).

This is an additional type that is sometimes (especially in the UK) used as a distinction from Type 2.

Instead of binding to cell surfaces, the antibodies recognise and bind to the cell surface receptors, which either prevents the intended ligand binding with the receptor or mimics the effects of the ligand, thus impairing cell signaling.

Some clinical examples:

- Graves' disease

- Myasthenia gravis

The use of Type 5 is rare. These conditions are more frequently classified as Type 2, though sometimes they are specifically segregated into their own subcategory of Type 2.

A food allergy is an abnormal immune response to food. The signs and symptoms may range from mild to severe. They may include itchiness, swelling of the tongue, vomiting, diarrhea, hives, trouble breathing, or low blood pressure. This typically occurs within minutes to several hours of exposure. When the symptoms are severe, it is known as anaphylaxis. Food intolerance and food poisoning are separate conditions.

Common foods involved include cow's milk, peanuts, eggs, shellfish, fish, tree nuts, soy, wheat, rice, and fruit. The common allergies vary depending on the country. Risk factors include a family history of allergies, vitamin D deficiency, obesity, and high levels of cleanliness. Allergies occur when immunoglobulin E (IgE), part of the body's immune system, binds to food molecules. A protein in the food is usually the problem. This triggers the release of inflammatory chemicals such as histamine. Diagnosis is usually based on a medical history, elimination diet, skin prick test, blood tests for food-specific IgE antibodies, or oral food challenge.

Early exposure to potential allergens may be protective. Management primarily involves avoiding the food in question and having a plan if exposure occurs. This plan may include giving adrenaline (epinephrine) and wearing medical alert jewelry. The benefits of allergen immunotherapy for food allergies is unclear, thus is not recommended as of 2015. Some types of food allergies among children resolve with age, including that to milk, eggs, and soy; while others such as to nuts and shellfish typically do not.

In the developed world, about 4% to 8% of people have at least one food allergy. They are more common in children than adults and appear to be increasing in frequency. Male children appear to be more commonly affected than females. Some allergies more commonly develop early in life, while others typically develop in later life. In developed countries, a large proportion of people believe they have food allergies when they actually do not have them.