The U.S Food and Drug Administration defines a serious adverse event as one when the patient outcome is one of the following:

- Death

- Life-threatening

- Hospitalization (initial or prolonged)

- Disability - significant, persistent, or permanent change, impairment, damage or disruption in the patient's body function/structure, physical activities or quality of life.

- Congenital anomaly

- Requires intervention to prevent permanent impairment or damage

Severity is a point on an arbitrary scale of intensity of the adverse event in question. The terms "severe" and "serious" when applied to adverse events are technically very different. They are easily confused but can not be used interchangeably, requiring care in usage.

A headache is severe, if it causes intense pain. There are scales like "visual analog scale" that help clinicians assess the severity. On the other hand, a headache is not usually serious (but may be in case of subarachnoid haemorrhage, subdural bleed, even a migraine may temporally fit criteria), unless it also satisfies the criteria for seriousness listed above.

Types A and B were proposed in the 1970s, and the other types were proposed subsequently when the first two proved insufficient to classify ADRs.

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

Idiosyncratic drug reactions, also known as type B reactions, are drug reactions that occur rarely and unpredictably amongst the population. This is not to be mistaken with idiopathic, which implies that the cause is not known. They frequently occur with exposure to new drugs, as they have not been fully tested and the full range of possible side-effects have not been discovered; they may also be listed as an adverse drug reaction with a drug, but are extremely rare.

Some patients have multiple-drug intolerance. Patients who have multiple idiopathic effects that are nonspecific are more likely to have anxiety and depression.

Idiosyncratic drug reactions appear to not be concentration dependent. A minimal amount of drug will cause an immune response, but it is suspected that at a low enough concentration, a drug will be less likely to initiate an immune response.

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.

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.

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.

The most common type of eruption is a morbilliform (resembling measles) or erythematous rash (approximately 90% of cases). Less commonly, the appearance may also be urticarial, papulosquamous, pustular, purpuric, bullous (with blisters) or lichenoid. Angioedema can also be drug-induced (most notably, by angiotensin converting enzyme inhibitors).

Some of the most severe and life-threatening examples of drug eruptions are erythema multiforme, Stevens–Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), hypersensitivity vasculitis, Drug induced hypersensitivity syndrome (DIHS), erythroderma and acute generalized exanthematous pustulosis (AGEP). These severe cutaneous drug eruptions are categorized as hypersensitivity reactions and are immune-mediated. There are four types of hypersensitivity reactions and many drugs can induce one or more hypersensitivity reactions.

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

The most common symptoms of salicylate sensitivity are:

- Stomach pain/upset stomach

- Tinnitus ringing of the ears

- Itchy skin, hives or rashes

- Asthma and other breathing difficulties

- Angioedema

- Headaches

- Swelling of hands, feet, eyelids, face and/or lips

- Bed wetting or urgency to pass water

- Persistent cough

- Changes in skin color/skin discoloration

- Fatigue

- Sore, itchy, puffy or burning eyes

- Sinusitis/Nasal polyps

- Diarrhea

- Nausea

- Hyperactivity

- Memory loss and poor concentration

- Depression

- Pseudoanaphylaxis

Salicylate sensitivity, also known as salicylate intolerance, is any adverse effect that occurs when a usual amount of salicylate is ingested. People with salicylate intolerance are unable to consume a normal amount of salicylate without adverse effects.

Salicylate sensitivity differs from salicylism, which occurs when an individual takes an overdose of salicylates. Salicylate overdose can occur in people without salicylate sensitivity, and can be deadly if untreated. For more information, see aspirin poisoning.

Salicylates are derivatives of salicylic acid that occur naturally in plants and serve as a natural immune hormone and preservative, protecting the plants against diseases, insects, fungi, and harmful bacteria. Salicylates can also be found in many medications, perfumes and preservatives. Both natural and synthetic salicylates can cause health problems in anyone when consumed in large doses. But for those who are salicylate intolerant, even small doses of salicylate can cause adverse reactions.

In adverse drug reactions involving overdoses, the toxic effect is simply an extension of the pharmacological effect (Type A adverse drug reactions). On the other hand, clinical symptoms of idiosyncratic drug reactions (Type B adverse drug reactions) are different from the pharmacological effect of the drug.

The proposed mechanism of most idiosyncratic drug reactions is immune-mediated toxicity. To create an immune response, a foreign molecule must be present that antibodies can bind to (i.e. the antigen) and cellular damage must exist. Very often, drugs will not be immunogenic because they are too small to induce immune response. However, a drug can cause an immune response if the drug binds a larger molecule. Some unaltered drugs, such as penicillin, will bind avidly to proteins. Others must be bioactivated into a toxic compound that will in turn bind to proteins. The second criterion of cellular damage can come either from a toxic drug/drug metabolite, or from an injury or infection.

These will sensitize the immune system to the drug and cause a response.

Idiosyncratic reactions fall conventionally under toxicology.

OAS sufferers may have any of a number of allergic reactions that usually occur very rapidly, within minutes of eating a trigger food. The most common reaction is an itching or burning sensation in the lips, mouth, ear canal, or pharynx. Sometimes other reactions can be triggered in the eyes, nose, and skin. Swelling of the lips, tongue, and uvula and a sensation of tightness in the throat may be observed. It can seldom result in anaphylaxis. If a sufferer swallows the food, and the allergen is not destroyed by the stomach acids, there is a good chance that there will be a reaction from histamine release later in the gastrointestinal tract. Vomiting, diarrhea, severe indigestion, or cramps may occur. Rarely, OAS may be severe and present as wheezing, vomiting, hives and low blood pressure.

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).

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.

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.

Alcohol-induced respiratory reactions, also termed alcohol-induced asthma and alcohol-induced respiratory symptoms, are increasingly recognized as a pathological bronchoconstriction response to the consumption of alcohol that afflicts many people with a "classical" form of asthma, the airway constriction disease evoked by the inhalation of allergens. Alcohol-induced respiratory reactions reflect the operation of different and often racially related mechanisms that differ from those of classical, allergen-induced asthma.

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 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.

Perfume intolerance or perfume allergy is a condition wherein people exhibit sensitivity or allergic reactions to ingredients in some perfumes and some other fragrances.

Oral allergy syndrome (OAS) is an allergic reaction in the mouth following eating food. It is a type of food allergy classified by a cluster of allergic reactions in the mouth in response to eating certain (usually fresh) fruits, nuts, and vegetables that typically develops in adults with hay fever.

OAS is perhaps the most common food-related allergy in adults. OAS is not a separate food allergy, but rather represents cross-reactivity between distant remnants of tree or weed pollen still found in certain fruits and vegetables. Therefore, OAS is only seen in people with seasonal pollen allergies, and mostly people who are allergic to tree pollen. It is usually limited to ingestion of only uncooked fruits or vegetables.

Another term used for this syndrome is pollen-food allergy. In adults up to 60% of all food allergic reactions are due to cross-reactions between foods and inhalative allergens.

OAS is a Type 1 or IgE-mediated immune response, which is sometimes called a "true allergy". The body's immune system produces IgE antibodies against pollen; in OAS, these antibodies also bind to (or cross-react with) other structurally similar proteins found in botanically related plants.

OAS can occur any time of the year but is most prevalent during the pollen season. Individuals with OAS usually develop symptoms within a few minutes after eating the food.

Symptoms depend on each person's allergies and each perfume's or fragrance's ingredients. Symptoms may include allergic contact dermatitis, asthma attacks, headaches, and others. The most common allergic reactions to perfume or fragrances added to products is contact dermatitis, though other symptoms may occur, including allergic conjunctivitis.

The diagnosis of the causal allergen is made by patch testing with a mixture of fragrance ingredients, the fragrance mix. This gives a positive patch-test reaction in about 10% of tested patients with eczema, and the most recent estimates show that 1.7–4.1% of the general population are sensitized to ingredients of the fragrance mix.

Two studies show that inhalant-like allergies and sensitivity/intolerances are experienced by a subset of the US population, in the form of asthma and chemical sensitivities. Results aggregated from both surveys found that 30.5% of the general population reported scented products on others irritating, 19% reported adverse health effects from air fresheners, and 10.9% reported irritation by scented laundry products vented outside.

Household products, such as soaps and detergents, perfume products, cosmetics, and other consumer goods, are estimated to use 2,500 different fragrance ingredients. Of those, approximately 100 different substances are known to elicit responses in at least some individuals. An estimated 1.7–4.1% of the general population shows a contact allergic response to a mix of common perfume ingredients.

The diagnosis is made by patch testing with a mixture of fragrance ingredients, the fragrance mix. This gives a positive patch-test reaction in about 10% of tested patients with eczema, and the most recent estimates show that 1.7–4.1% of the general population are sensitized to ingredients of the fragrance mix.

Although products can be labeled "fragrance-free", many still contain lesser-known fragrance chemicals that consumers may not recognize.

Cinnamaldehyde (cinnamic aldehyde) is a common fragrance allergen.

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.

Pseudocholinesterase deficiency is an inherited blood plasma enzyme abnormality in which the body's production of butyrylcholinesterase (BCHE; pseudocholinesterase) is impaired. People who have this abnormality may be sensitive to certain anesthetic drugs, including the muscle relaxants succinylcholine and mivacurium as well as other ester local anesthetics.