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.

Antineoplastic resistance, often used interchangeably with chemotherapy resistance, is the multiple drug resistance of neoplastic (cancerous) cells, or the ability of cancer cells to survive and grow despite anti-cancer therapies.

There are two general causes of antineoplastic therapy failure: Inherent genetic characteristics, giving cancer cells their resistance, which is rooted in the concept of cancer cell heterogeneity and acquired resistance after drug exposure. Altered membrane transport, enhanced DNA repair, apoptotic pathway defects, alteration of target molecules, protein and pathway mechanisms, such as enzymatic deactivation.

Since cancer is a genetic disease, two genomic events underlie acquired drug resistance: Genome alterations (e.g. gene amplification and deletion) and epigenetic modifications.

Cancer cells are constantly using a variety of tools, involving genes, proteins and altered pathways, to ensure their survival against antineoplastic drugs.

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.

Anticonvulsant/sulfonamide hypersensitivity syndrome is a potentially serious hypersensitivity reaction that can be seen with drugs with an aromatic amine chemical structure, such as aromatic anticonvulsants (e.g. diphenylhydantoin, phenobarbital, phenytoin, carbamazepine, lamotrigine), sulfonamides, or other drugs with an aromatic amine (procainamide). Cross-reactivity should not occur between drugs with an aromatic amine and drugs without an aromatic amine (e.g., sulfonylureas, thiazide diuretics, furosemide, and acetazolamide); therefore, these drugs can be safely used in the future.

The hypersensitivity syndrome is characterized by a skin eruption that is initially morbilliform. The rash may also be a severe Stevens-Johnson syndrome or toxic epidermal necrolysis. Systemic manifestations occur at the time of skin manifestations and include eosinophilia, hepatitis, and interstitial nephritis. However, a subgroup of patients may become hypothyroid as part of an autoimmune thyroiditis up to 2 months after the initiation of symptoms.

This kind of adverse drug reaction is caused by the accumulation of toxic metabolites; it is not the result of an IgE-mediated reaction. The risk of first-degree relatives’ developing the same hypersensitivity reaction is higher than in the general population.

As this syndrome can present secondary to multiple anticonvulsants, the general term "anticonvulsant hypersensitivity syndrome" is favored over the original descriptive term "dilantin hypersensitivity syndrome."

Cytokine release syndrome is an adverse effect of some monoclonal antibody drugs, as well as adoptive T-cell therapies. Severe cases have been called "cytokine storms", a term borrowed from discussions of the pathophysiology of immune disorders and infectious disease.

CRS has been known since the approval of the first monoclonal antibody drug, Muromonab-CD3, which causes CRS, but people working in the field of drug development at biotech and pharmaceutical companies, regulatory agencies, and academia began to more intensely discuss methods to classify it and how to mitigate its risk following the disastrous 2006 Phase I clinical trial of TGN 1412, in which the six subjects experienced severe CRS.

Antineoplastic resistance, synonymous with chemotherapy resistance, is the ability of cancer cells to survive and grow despite different anti-cancer therapies, i.e. their multiple drug resistance. There are two general causes of antineoplastic therapy failure:

Inherent resistance, such as genetic characteristics, giving cancer cells their resistance from the beginning, which is rooted in the concept of cancer cell heterogeneity and acquired resistance after drug exposure.

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).

CRS is an adverse effect of some drugs and is a form of systemic inflammatory response syndrome.

The Common Terminology Criteria for Adverse Events classifications for CRS as of version 4.03 issued in 2010 were:

Intolerance to analgesics, particularly NSAIDs, is relatively common. It is thought that a variation in the metabolism of arachidonic acid is responsible for the intolerance. Symptoms include chronic rhinosinusitis with nasal polyps, asthma, gastrointestinal ulcers, angioedema, and urticaria.

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.

A drug interaction is a situation in which a substance (usually another drug) affects the activity of a drug when both are administered together. This action can be synergistic (when the drug's effect is increased) or antagonistic (when the drug's effect is decreased) or a new effect can be produced that neither produces on its own. Typically, interactions between drugs come to mind (drug-drug interaction). However, interactions may also exist between drugs and foods (drug-food interactions), as well as drugs and medicinal plants or herbs (drug-plant interactions). People taking antidepressant drugs such as monoamine oxidase inhibitors should not take food containing tyramine as hypertensive crisis may occur (an example of a drug-food interaction). These interactions may occur out of accidental misuse or due to lack of knowledge about the active ingredients involved in the relevant substances.

It is therefore easy to see the importance of these pharmacological interactions in the practice of medicine. If a patient is taking two drugs and one of them increases the effect of the other it is possible that an overdose may occur. The interaction of the two drugs may also increase the risk that side effects will occur. On the other hand, if the action of a drug is reduced it may cease to have any therapeutic use because of under dosage. Notwithstanding the above, on occasion these interactions may be sought in order to obtain an improved therapeutic effect. Examples of this include the use of codeine with paracetamol to increase its analgesic effect. Or the combination of clavulanic acid with amoxicillin in order to overcome bacterial resistance to the antibiotic. It should also be remembered that there are interactions that, from a theoretical standpoint, may occur but in clinical practice have no important repercussions.

The pharmaceutical interactions that are of special interest to the practice of medicine are primarily those that have negative effects for an organism. The risk that a pharmacological interaction will appear increases as a function of the number of drugs administered to a patient at the same time. Over a third (36%) of older adults in the U.S. regularly use 5 or more medications or supplements and 15% are potentially at risk for a major drug-drug interaction. Both the use of medications and subsequent adverse drug interactions have increased significantly between 2005-2011.

It is possible that an interaction will occur between a drug and another substance present in the organism (i.e. foods or alcohol). Or in certain specific situations a drug may even react with itself, such as occurs with dehydration. In other situations, the interaction does not involve any effect on the drug. In certain cases, the presence of a drug in an individual's blood may affect certain types of laboratory analysis (analytical interference).

It is also possible for interactions to occur outside an organism before administration of the drugs has taken place. This can occur when two drugs are mixed, for example, in a saline solution prior to intravenous injection. Some classic examples of this type of interaction include that thiopentone and suxamethonium should not be placed in the same syringe and same is true for benzylpenicillin and heparin. These situations will all be discussed under the same heading due to their conceptual similarity.

Drug interactions may be the result of various processes. These processes may include alterations in the pharmacokinetics of the drug, such as alterations in the absorption, distribution, metabolism, and excretion (ADME) of a drug. Alternatively, drug interactions may be the result of the pharmacodynamic properties of the drug, e.g. the co-administration of a receptor antagonist and an agonist for the same receptor.

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.

SJS usually begins with fever, sore throat, and fatigue, which is commonly misdiagnosed and therefore treated with antibiotics. SJS and TEN are often heralded by fever, sore throat, cough, and burning eyes for 1 to 3 days. Patients with SJS and TEN frequently experience burning pain of their skin at the start of disease. Ulcers and other lesions begin to appear in the mucous membranes, almost always in the mouth and lips, but also in the genital and anal regions. Those in the mouth are usually extremely painful and reduce the patient's ability to eat or drink. Conjunctivitis of the eyes occurs in about 30% of children who develop SJS. A rash of round lesions about an inch across arises on the face, trunk, arms and legs, and soles of the feet, but usually not the scalp.

Drug-induced fever is a symptom of an adverse drug reaction wherein the administration of drugs intended to help a patient causes a hypermetabolic state resulting in fever. The drug may interfere with heat dissipation peripherally, increase the rate of metabolism, evoke a cellular or humoral immune response, mimic endogenous pyrogen, or damage tissues.

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

Drug intolerance or drug sensitivity refers to an inability to tolerate the adverse effects of a medication, generally at therapeutic or subtherapeutic doses. Conversely, a patient is said to be "tolerating" a drug when they can tolerate its adverse effects. It is not to be confused with a drug allergy, which is a form of drug intolerance, but requires an immune-mediated component. It is also not to be confused with drug tolerance ("drug resistance," or tachyphylaxis) which refers to a "lack" of adverse effects even at higher than average doses. Some instances of drug intolerance are known to result from genetic variations in drug metabolism.

The side effects of penicillin are bodily responses to penicillin and closely related antibiotics that do not relate directly to its effect on bacteria. A side effect is an effect that is not intended with normal dosaging. Some of these reactions are visible and some occur in the body's organs or blood. Penicillins are a widely-used group of medications that are effective for the treatment of a wide variety of bacterial infections in human adults and children as well as other species. Some side effects are predictable, of which some are common but not serious, some are uncommon and serious and others are rare. The route of administration of penicillin can have an effect on the development of side effects. An example of this is irritation and inflammation that develops at a peripheral infusion site when penicillin is administered intravenously. In addition, penicillin is available in different forms. There are different penicillin medications (penicillin G benzathine, penicillin G potassium, penicillin G procaine, and penicillin V) as well as a number of β-lactam antibiotics derived from penicillin (e.g. amoxicillin) generally also referred to as "penicillin".

Side effects may only last for a short time and then go away. Side effects can be relieved in some cases with non pharmacological treatment. Some side effects require treatment to correct potentially serious and sometimes fatal reactions to penicillin. Penicillin has not been found to cause birth defects.

Common adverse drug reactions (≥ 1% of people) associated with use of the penicillins include diarrhoea, hypersensitivity, nausea, rash, neurotoxicity, urticaria, and superinfection (including candidiasis). Infrequent adverse effects (0.1–1% of people) include fever, vomiting, erythema, dermatitis, angioedema, seizures (especially in people with epilepsy), and pseudomembranous colitis.

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

The symptoms can occur anywhere between days to months after administration of the offending medication, depending on the dose and speed of administration (Baack and Burgdorf, 1991; Demirçay, 1997). The patient first experiences tingling and/or numbness of the palms and soles that evolves into painful, symmetric, and well-demarcated swelling and red plaques. This is followed by peeling of the skin and resolution of the symptoms (Apisarnthanarax and Duvic 2003).

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

Toxic abortion is a medical phenomenon of spontaneous abortion, miscarriage, or stillbirth caused by toxins in the environment of the mother during pregnancy, especially as caused by toxic environmental pollutants, though sometimes reported as caused by naturally occurring plant toxins.

Chemotherapy-induced acral erythema (also known as palmar-plantar erythrodysesthesia, palmoplantar erythrodysesthesia, or hand-foot syndrome) is reddening, swelling, numbness and desquamation (skin sloughing or peeling) on palms of the hands and soles of the feet (and, occasionally, on the knees, elbows, and elsewhere) that can occur after chemotherapy in patients with cancer. Hand-foot syndrome is also rarely seen in sickle-cell disease. These skin changes usually are well demarcated. Acral erythema typically disappears within a few weeks after discontinuation of the offending drug.

Stevens–Johnson syndrome (SJS) is a type of severe skin reaction. Together with toxic epidermal necrolysis (TEN) it forms a spectrum of disease, with SJS being less severe. Early symptoms include fever and flu-like symptoms. A few days later the skin begins to blister and peel forming painful raw areas. Mucous membranes, such as the mouth, are also typically involved. Complications include dehydration, sepsis, pneumonia, and multiple organ failure.

The most common cause is certain medications such as lamotrigine, carbamazepine, allopurinol, sulfonamide antibiotics, and nevirapine. Other causes can include infections such as "Mycoplasma pneumoniae" and cytomegalovirus or the cause may remain unknown. Risk factors include HIV/AIDS and systemic lupus erythematosus. The diagnosis is based on involvement of less than 10% of the skin. It is known as TEN when more than 30% of the skin is involved and an intermediate form with 10 to 30% involvement. Erythema multiforme (EM) is generally considered a separate condition.

Treatment typically takes place in hospital such as in a burn unit or intensive care unit. Efforts may include stopping the cause, pain medication, antihistamines, antibiotics, intravenous immunoglobulins, or corticosteroids. Together with TEN it affects 1 to 2 people per million per year. It is twice as common in males as females. Typical onset is under the age of 30. Skin usually regrows over two to three weeks; however, complete recovery can take months.

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.