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.

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.

A study by the Agency for Healthcare Research and Quality (AHRQ) found that in 2011, sedatives and hypnotics were a leading source for adverse drug events seen in the hospital setting. Approximately 2.8% of all ADEs present on admission and 4.4% of ADEs that originated during a hospital stay were caused by a sedative or hypnotic drug. A second study by AHRQ found that in 2011, the most common specifically identified causes of adverse drug events that originated during hospital stays in the U.S. were steroids, antibiotics, opiates/narcotics, and anticoagulants. Patients treated in urban teaching hospitals had higher rates of ADEs involving antibiotics and opiates/narcotics compared to those treated in urban nonteaching hospitals. Those treated in private, nonprofit hospitals had higher rates of most ADE causes compared to patients treated in public or private, for-profit hospitals.

In the U.S., females had a higher rate of ADEs involving opiates and narcotics than males in 2011, while male patients had a higher rate of anticoagulant ADEs. Nearly 8 in 1,000 adults aged 65 years or older experienced one of the four most common ADEs (steroids, antibiotics, opiates/narcotics, and anticoagulants) during hospitalization. A study showed that 48% of patients had an adverse drug reaction to at least one drug, and pharmacist involvement helps to pick up adverse drug reactions.

In 2012 McKinsey &Co. concluded that the cost of the 35 million preventable adverse drug events would be as high as US$115 billion.

The primary treatment strategy is to eliminate or discontinue the offensive agent. Supportive therapy, such as ice packs, may be provided to get the body temperature within physiologic range. In severe cases, when the fever is high enough (generally at or above ~104° F or 40° C), aggressive cooling such as an ice bath and pharmacologic therapy such as benzodiazepines may be deemed appropriate.

Many countries have official bodies that monitor drug safety and reactions. On an international level, the WHO runs the Uppsala Monitoring Centre, and the European Union runs the European Medicines Agency (EMEA). In the United States, the Food and Drug Administration (FDA) is responsible for monitoring post-marketing studies.

In Canada, the Marketed Health Products Directorate of Health Canada is responsible for the surveillance of marketed health products. In Australia, the Therapeutic Goods Administration (TGA) conducts postmarket monitoring of therapeutic products.

When penicillin is used at high doses hypokalemia, metabolic acidosis, and hyperkalemia can occur. Developing hypernatremia after administering high doses of penicillin can be a serious side effect.

Among US adults older than 55, 4% are taking medication and or supplements that put them at risk of a major drug interaction. Potential drug-drug interactions have increased over time and are more common in the low educated elderly even after controlling for age, sex, place of residence, and comorbidity.

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.

The side effects of penicillin can be altered by taking other medications at the same time. Taking oral contraceptives along with penicillin may lower the effects of the contraceptive. When probenecid is used concurrently with penicillin, kidney excretion of probenecid is increase resulting in higher blood levels of penicillin in the circulation. In some instances, this would be intended therapeutic effect. In other instances, this is an unintended side effect. Neomycin can lower the absorption of penicillin from the gastrointestinal tract resulting in lower than expected levels of penicillin in the circulation. This side effect may result in an ineffective therapeutic effect of penicillin. When methotrexate is administered with penicillin, toxicity may occur related to methotrexante.

The culprit can be both a prescription drug or an over-the-counter medication.

Examples of common drugs causing drug eruptions are antibiotics and other antimicrobial drugs, sulfa drugs, nonsteroidal anti-inflammatory drugs (NSAIDs), biopharmaceuticals, chemotherapy agents, anticonvulsants, and psychotropic drugs. Common examples include photodermatitis due to local NSAIDs (such as piroxicam) or due to antibiotics (such as minocycline), fixed drug eruption due to acetaminophen or NSAIDs (Ibuprofen), and the rash following ampicillin in cases of mononucleosis.

Certain drugs are less likely to cause drug eruptions (rates estimated to be ≤3 per 1000 patients exposed). These include: digoxin, aluminum hydroxide, multivitamins, acetaminophen, bisacodyl, aspirin, thiamine, prednisone, atropine, codeine, hydrochlorothiazide, morphine, insulin, warfarin, and spironolactone.

The detection of laboratory parameters is based on physicochemical reactions between the substance being measured and reagents designed for this purpose. These reactions can be altered by the presence of drugs giving rise to an over estimation or an underestimation of the real results. Levels of cholesterol and other blood lipids can be overestimated as a consequence of the presence in the blood of some psychotropic drugs. These overestimates should not be confused with the action of other drugs that actually increase blood cholesterol levels due to an interaction with its metabolism.

Most experts consider that these are not true interactions, so they will not be dealt with further in this discussion.

These chemical reactions are also known as pharmacological incompatibilities. The reactions occur when two or more drugs are mixed outside the body of the organism for the purpose of joint administration. Usually the interaction is antagonistic and it almost always affects both drugs. Examples of these types of interactions include the mixing of penicillins and aminoglycosides in the same serum bottle, which causes the formation of an insoluble precipitate, or the mixing of ciprofloxacin with furosemide. The interaction of some drugs with the transport medium can also be included here. This means that certain drugs cannot be administered in plastic bottles because they bind with the bottle's walls, reducing the drug's concentration in solution.

Many authors do not consider them to be interactions in the strictest sense of the word. An example is the database of the General Council of Official Pharmacists Colleges of Spain (Consejo General de Colegios Oficiales de Farmacéuticos de España), that does not include them among the 90,000 registered interactions.

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.

The cooling of hands and feet during chemotherapy may help prevent PPE (Baack and Burgdorf, 1991; Zimmerman et al., 1995). Support for this and a variety of other approaches to treat or prevent acral erythema comes from small clinical studies, although none has been proven in a randomised controlled clinical trial of sufficient size.

The main treatment for acral erythema is discontinuation of the offending drug, and symptomatic treatment to provide analgesia, lessen edema, and prevent superinfection. However, the treatment for the underlying cancer of the patient must not be neglected. Often, the discontinued drug can be substituted with another cancer drug or cancer treatment.

Symptomatic treatment can include wound care, elevation, and pain medication. Corticosteroids and pyridoxine have also been used to relieve symptoms. Other studies do not support the conclusion.

A number of additional remedies are listed in recent medical literature. Among them henna and 10% uridine ointment which went through clinical trial.

It is estimated that 2—3 percent of hospitalised patients are affected by a drug eruption, and that serious drug eruptions occur in around 1 in 1000 patients.

Drugs in systemic circulation have a certain concentration in the blood, which serves as a surrogate marker for how much drug will be delivered throughout the body (how much drug the rest of the body will "see"). There exists a minimum concentration of drug within the blood that will give rise to the intended therapeutic effect (minimum effective concentration, MEC), as well as a minimum concentration of drug that will give rise to an unintended adverse drug event (minimum toxic concentration, MTC). The difference between these two values is generally referred to as the therapeutic window. Different drugs have different therapeutic windows, and different people will have different MECs and MTCs for a given drug. If someone has a very low MTC for a drug, they are likely to experience adverse effects at drug concentrations lower than what it would take to produce the same adverse effects in the general populace; thus, the individual will experience significant toxicity at a dose that is otherwise considered "normal" for the average person. This individual will be considered "intolerant" to that drug.

There are a variety of factors that can affect the MTC, which is often the subject of clinical pharmacokinetics. Variations in MTC can occur at any point in the ADME (absorption, distribution, metabolism, and excretion) process. For example, a patient could possess a genetic defect in a drug metabolizing enzyme in the cytochrome P450 superfamily. While most individuals will possess the effective metabolizing machinery, a person with a defect will have a difficult time trying to clear the drug from their system. Thus, the drug will accumulate within the blood to higher-than-expected concentrations, reaching a MTC at a dose that would otherwise be considered normal for the average person. In other words, in a person that is intolerant to a medication, it is possible for a dose of 10 mg to "feel" like a dose of 100 mg, resulting in an overdose—a "normal" dose can be a "toxic" dose in these individuals, leading to clinically significant effects.

There is also an aspect of drug intolerance that is subjective. Just as different people have different pain tolerances, so too do people have different tolerances for dealing with the adverse effects from their medications. For example, while opioid-induced constipation may be tolerable to some individuals, other people may stop taking an opioid due to the unpleasantness of the constipation even if it brings them significant pain relief.

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:

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 term "toxic abortion" was first used to identify this phenomenon in humans in the earliest studies of the effects of pollutants on pregnancy in 1928, "An Experimental Investigation Concerning Toxic Abortion Produced by Chemical Agents" by Morris M. Datnow M.D.

Toxic abortion chemicals studied at that time were:

Petrochemicals,

Heavy metals,

Organic solvents,

Tetrachloroethylene,

Glycol ethers,

2-Bromopropane,

Ethylene oxide,

Anesthetic gases, and

Antineoplastic drugs.

In 1932, the "Journal of State Medicine" reported on a natural variation, with the occurrence of "a considerable number of cases of toxic abortion" being caused by untreated dental caries.

Study of pollution-caused abortion in humans ceased for a considerable time, interest renewing in the 2000s. A 2009 study found that fossil fuels play a role, as "pregnant African-American women who live within a half mile of freeways and busy roads were three times more likely to have miscarriages than women who don't regularly breathe exhaust fumes." A 2011 study found a correlation between exposure to workplace toxins and spontaneous abortion, and called for further study. "Newsweek" magazine reported in May 2014 that a spike in stillborn babies in the town of Vernal, in Utah, had correlated with an increase in pollution from new gas and oil drilling. "Newsweek" reported that "Vernal’s rate of neonatal mortality appears to have climbed from about average in 2010 (relative to national figures) to six times the normal rate three years later." "Newsweek" quoted one expert's observation that "We know that pregnant women who breathe more air pollution have much higher rates of virtually every adverse pregnancy outcome that exists." A study published in the "Journal of Environmental Health" in October 2014 found tetrachloroethylene or PCE, to be "linked to increased risk for stillbirths and other pregnancy complications."

The PCE study found that "pregnancies with high exposure to PCE were 2.4 times more likely to end with stillborn babies and 1.4 times more likely to experience placental abruption — when the placenta peels away from uterine wall before delivery, causing the mother to bleed and the baby to lose oxygen — compared with pregnancies never exposed to PCE." Higher exposure lead to a 35 percent higher risk of abruption. PCE has also been tied to an increased risk for cancer. Children exposed to PCE as fetuses and toddlers are more likely to use drugs later in life. The toxin has been linked to mental illness, an increased risk of breast cancer and some birth defects. It has been tied to anxiety, depression, and impairments in cognition, memory and attention. PCE contamination has been found in the Massachusetts water supply and "on military bases across the country," and "water systems in California and Pennsylvania and have also been found to be contaminated with PCE."

In 2015, "Newsweek" reported that chemicals found in fast food wrappers multiply miscarriage risk by sixteen times.

Some instances have been reported of women intentionally seeking to induce toxic abortion, where circumstances make medical abortion difficult to obtain, by exposing themselves to environmental toxins.

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."

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.

Drug allergies are attributed to "drug hypersensitivity," otherwise known as objectively reproducible symptoms or signs initiated by exposure to a drug at a dose normally tolerated by non-hypersensitive persons. Drug hypersensitivity reactions are the mediators of a drug allergy.

There are two mechanisms for a drug allergy to occur: IgE or non-IgE mediated. In IgE-mediated reactions, also known as Immunoglobulin E mediated reactions, drug allergens bind to IgE antibodies, which are attached to mast cells and basophils, resulting in IgE cross-linking, cell activation and release of preformed and newly formed mediators.

Although SJS can be caused by viral infections and malignancies, the main cause is medications. A leading cause appears to be the use of antibiotics, particularly sulfa drugs. Between 100 and 200 different drugs may be associated with SJS. No reliable test exists to establish a link between a particular drug and SJS for an individual case. Determining what drug is the cause is based on the time interval between first use of the drug and the beginning of the skin reaction. Drugs discontinued more than 1 month prior to onset of mucocutaneous physical findings are highly unlikely to cause SJS and TEN. SJS and TEN most often begin between 4 and 28 days after culprit drug administration. A published algorithm (ALDEN) to assess drug causality gives structured assistance in identifying the responsible medication.

SJS may be caused by adverse effects of the drugs vancomycin, allopurinol, valproate, levofloxacin, diclofenac, etravirine, isotretinoin, fluconazole, valdecoxib, sitagliptin, oseltamivir, penicillins, barbiturates, sulfonamides, phenytoin, azithromycin, oxcarbazepine, zonisamide, modafinil, lamotrigine, nevirapine, pyrimethamine, ibuprofen, ethosuximide, carbamazepine, bupropion, telaprevir, and nystatin.

Medications that have traditionally been known to lead to SJS, erythema multiforme, and toxic epidermal necrolysis include sulfonamide antibiotics, penicillin antibiotics, cefixime (antibiotic), barbiturates (sedatives), lamotrigine, phenytoin (e.g., Dilantin) (anticonvulsants) and trimethoprim. Combining lamotrigine with sodium valproate increases the risk of SJS.

Nonsteroidal anti-inflammatory drugs (NSAIDs) are a rare cause of SJS in adults; the risk is higher for older patients, women, and those initiating treatment. Typically, the symptoms of drug-induced SJS arise within a week of starting the medication. Similar to NSAIDs, paracetamol (acetaminophen) has also caused rare cases of SJS. People with systemic lupus erythematosus or HIV infections are more susceptible to drug-induced SJS.

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.

The 1997 International Germ Cell Consensus Classification is a tool for estimating the risk of relapse after treatment of malignant germ cell tumor.

A small study of ovarian tumors in girls reports a correlation between cystic and benign tumors and, conversely, solid and malignant tumors. Because the cystic extent of a tumor can be estimated by ultrasound, MRI, or CT scan before surgery, this permits selection of the most appropriate surgical plan to minimize risk of spillage of a malignant tumor.

Access to appropriate treatment has a large effect on outcome. A 1993 study of outcomes in Scotland found that for 454 men with non-seminomatous (non-germinomatous) germ cell tumors diagnosed between 1975 and 1989, 5-year survival increased over time and with earlier diagnosis. Adjusting for these and other factors, survival was 60% higher for men treated in a cancer unit that treated the majority of these men, even though the unit treated more men with the worst prognosis.

Choriocarcinoma of the testicles has the worst prognosis of all germ cell cancers