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.

6-Pyruvoyltetrahydropterin synthase deficiency is an autosomal recessive disorder that causes malignant hyperphenylalaninemia due to tetrahydrobiopterin deficiency.

It belongs to the rare diseases. It is a recessive disorder that is accompanied by hyperphenylalaninemia. Commonly reported symptoms are initial truncal hypotonia, subsequent appendicular hypertonia, bradykinesia, cogwheel rigidity, generalized dystonia, and marked diurnal fluctuation. Other reported clinical features include difficulty in swallowing, oculogyric crises, somnolence, irritability, hyperthermia, and seizures. Chorea, athetosis, hypersalivation, rash with eczema, and sudden death have also been reported. Patients with mild phenotypes may deteriorate if given folate antagonists such as methotrexate, which can interfere with a salvage pathway through which dihydrobiopterin is converted into tetrahydrobiopterin via dihydrofolate reductase. Treatment options include substitution with neurotransmitter precursors (levodopa, 5-hydroxytryptophan), monoamine oxidase inhibitors, and tetrahydrobiopterin. Response to treatment is variable and the long-term and functional outcome is unknown. To provide a basis for improving the understanding of the epidemiology, genotype/phenotype correlation and outcome of these diseases their impact on the quality of life of patients, and for evaluating diagnostic and therapeutic strategies a patient registry was established by the noncommercial International Working Group on Neurotransmitter Related Disorders (iNTD).

Some fruit juices and fruits can interact with numerous drugs, in many cases causing adverse effects. The effect was first discovered by accident, when a test of drug interactions with alcohol used grapefruit juice to hide the taste of the ethanol.

It is still best-studied with grapefruit and grapefruit juice, but similar effects have more recently been seen with some (not all) other citrus fruits. One medical review advises patients to avoid all citrus juices until further research clarifies the risks. The interacting chemicals are found in many plants, and so many other foods may be affected; effects have been observed with apple juice, but their clinical significance is not yet known.

Normal amounts of food and drink, such as one whole grapefruit or a small glass () of grapefruit juice, can cause drug overdose toxicity. Fruit consumed three days before the medicine can still have an effect. The relative risks of different types of citrus fruit have not been systematically studied. Affected drugs typically have an auxiliary label saying “Do not take with grapefruit” on the container, and the interaction is elaborated on in the package insert. People are also advised to ask their physician or pharmacist about drug interactions.

The effects are caused by furanocoumarins (and, to a lesser extent, flavonoids). These chemicals inhibit key drug metabolizing enzymes, such as cytochrome P450 3A4 (CYP3A4). CYP3A4 is a metabolizing enzyme for almost 50% of drugs, and is found in the liver and small intestinal epithelial cells. As a result, many drugs are affected. Inhibition of enzymes can have two different effects, depending on whether the drug is either

1. metabolized by the enzyme to an inactive metabolite, "or"

2. activated by the enzyme to an active metabolite.

If the active drug is metabolized by the inhibited enzyme, then the fruit will stop the drug being metabolized, leaving elevated concentrations of the medication in the body, which can cause adverse effects. Conversely, if the medication is a prodrug, it needs to be metabolised to be converted to the active drug. Compromising its metabolism lowers concentrations of the active drug, reducing its therapeutic effect, and risking therapeutic failure.

Low drug concentrations can also be caused when the fruit suppresses drug absorption from the intestine.

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.

Patients affected by ADT tachyphylaxis experience a noticeably sudden progressive decrease in response to SSRIs. The reported rates of this condition vary from 9% to 33% of SSRI users, and the majority of those affected are less responsive to subsequent treatments. In most observational studies, these individuals suffer a recurrence or relapse of depression without changing the previously effective dose.

ADT tachyphylaxis incorporates drug sensitivity as a potential causal factor for the decreased response. However, tolerance provides a more accurate explanation. While the exact cause of ADT tachyphylaxis in individual cases is unknown, drug tolerance is a more comprehensive model, as it includes mechanisms of pharmacodynamic tolerance, metabolic tolerance, and others.

The effect of grapefruit juice with regard to drug absorption was originally discovered in 1989. The first published report on grapefruit drug interactions was in 1991 in the Lancet entitled "Interactions of Citrus Juices with Felodipine and Nifedipine," and was the first reported food-drug interaction clinically. However, the effect only became well-publicized after being responsible for a number of bad interactions with medication.

Drug-induced angioedema is a known complication of the use of angiotensin-converting enzyme (ACE) inhibitors, angiotensin II antagonists (ARBs), and Angiotensin-Neprilysin Inhibitor LCZ969. The angioedema appears to be dose dependent as it may resolve with decreased dose.

Some common ACE Inhibitors are:

- Benazepril (Lotensin)

- Captopril (Capoten)

- Enalapril (Vasotec)

- Lisinopril (Prinivil, Zestril)

- Ramipril (Altace)

Some common ARBs are:

- Candesartan (Atacand)

- Losartan (Cozaar)

- Olmesartan (Benicar)

- Valsartan (Diovan)

Angioedema presents itself as an abrupt onset of non-pitting, non-itchy swelling that involves the mucosal layers. Some common locations of angioedema are the face, particularly the lips and around the eyes, hands and feet, and genitalia. A rare, yet serious complication is one inside the abdomen, the symptom usually being severe stomach upset, which is much less obvious than the other locations.

The chance of drug-induced angioedema is extremely uncommon, however, as studies show incidence of less than 1%. The reason this adverse effect may occur is due to the build-up of bradykinin, a vasodilator. This causes blood vessels to dilate and allow for fluid buildup in the mucosal surfaces.

Antidepressant treatment tachyphylaxis (ADT tachyphylaxis), also known as Prozac poop-out, is a medical condition in which progressive or acute tolerance effects are seen following chronic administration of a drug. ADT tachyphylaxis specifically refers to a sudden decrease in response to selective serotonin reuptake inhibitors (SSRIs), which are the most commonly prescribed antidepressants. Although less commonly prescribed as antidepressants (having lost popularity following the introduction of SSRIs), monoamine oxidase inhibitors, or MAOIs, have also incurred a "poop-out" effect among depressed patients.

M2 is a subtype of AML (Acute Myeloid Leukemia).

It is also known as "Acute Myeloblastic Leukemia with Maturation".

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.

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).

Gene expression profiling has revealed that diffuse large B-cell lymphoma (DLBCL) is composed of at least 3 different sub-groups, each having distinct oncogenic mechanisms that respond to therapies in different ways. Germinal Center B-Cell like (GCB) DLBCLs appear to arise from normal germinal center B cells, while Activated B-cell like (ABC) DLBCLs are thought to arise from postgerminal center B cells that are arrested during plasmacytic differentiation. The differences in gene expression between GCB DLBCL and ABC DLBCL are as vast as the differences between distinct types of leukemia, but these conditions have historically been grouped together and treated as the same disease.

Clostridial necrotizing enteritis (CNE), also called enteritis necroticans and pigbel, is an often fatal type of food poisoning caused by a β-toxin of "Clostridium perfringens", Type C. It occurs in some developing countries, but was also documented in Germany following World War II. The toxin is normally inactivated by certain proteolytic enzymes and by normal cooking, but when these protections are impeded, the disease emerges.

Apitoxin, or honey bee venom, is a bitter colorless liquid containing proteins, which may produce local inflammation. It may have similarities to sea nettle toxin.

Antidepressant discontinuation syndrome is a condition that can occur following the interruption, dose reduction, or discontinuation of antidepressant drugs, including selective serotonin re-uptake inhibitors (SSRIs) or serotonin–norepinephrine reuptake inhibitors (SNRIs). The symptoms can include flu-like symptoms and disturbances in sleep, senses, movement, mood, and thinking. In most cases symptoms are mild, short-lived, and resolve without treatment. More severe cases may be successfully treated by reintroduction of the drug, provided reintroduction is done in a timely fashion. Symptoms, including tardive akathisia, and Post SSRI Sexual Dysfunction (PSSD) may persist for months to years, yet may spontaneously resolve after prolonged presence.

Clonal hypereosinophilia, also termed Primary hypereosinophelia or clonal eosinophilia, is a grouping of hematological disorder characterized by the development and growth of a pre-malignant or malignant population of eosinophils, a type of white blood cell, in the bone marrow, blood, and/or other tissues. This population consists of a clone of eosinophils, i.e. a group of genetically identical eosinophils derived from a sufficiently mutated ancestor cell.

The clone of eosinophils bear a mutation in any one of several genes that code for proteins that regulate cell growth. The mutations cause these proteins to be continuously active and thereby to stimulate growth in an uncontrolled and continuous manner. The expanding population of eosinophils, initially formed in the bone marrow may spread to the blood and then enter into and injure various tissues and organs.

Clinically, clonal eosinophilia resembles various types of chronic or acute leukemias, lymphomas, or myeloproliferative hematological malignancies. However, many of the clonal hypereosinophilias are distinguished from these other hematological malignancies by the genetic mutations which underlie their development and, more importantly, by their susceptibility to specific treatment regiments. That is, many types of these disorders are remarkably susceptible to relatively non-toxic drugs.

HIV-associated lipodystrophy commonly presents with fat loss in face, buttocks, arms and legs.

There is also fat "accumulation" in various body parts. Patients often present with "buffalo hump"-like fat deposits in their upper backs. Breast size of patients (both male and female) tends to increase. In addition, patients develop abdominal obesity.

People with discontinuation syndrome have been on an antidepressant for at least four weeks and have recently stopped taking the medication, whether abruptly, after a fast taper, or each time the medication is reduced on a slow taper. Commonly reported symptoms include flu-like symptoms (nausea, vomiting, diarrhea, headaches, sweating) and sleep disturbances (insomnia, nightmares, constant sleepiness). Sensory and movement disturbances have also been reported, including imbalance, tremors, vertigo, dizziness, and electric-shock-like experiences in the brain, often described by sufferers as "brain zaps". Mood disturbances such as dysphoria, anxiety, or agitation are also reported, as are cognitive disturbances such as confusion and hyperarousal.

In cases associated with sudden discontinuation of MAO inhibitors, acute psychosis has been observed. Over fifty symptoms have been reported.

Most cases of discontinuation syndrome last between one and four weeks, are relatively mild, and resolve on their own; in rare cases symptoms can be severe or extended. Paroxetine ("Paxil") and venlafaxine ("Effexor") seem to be particularly difficult to discontinue and prolonged withdrawal syndrome lasting over 18 months have been reported with paroxetine.

A 2009 Advisory Committee to the FDA found that online anecdotal reports of discontinuation syndrome related to duloxetine ("Cymbalta") included severe symptoms and exceeded prevalence of both paroxetine ("Paxil") and venlafaxine ("Effexor") reports by over 250% (although acknowledged this may have been influenced by duloxetine being a much newer drug). It also found that the safety information provided by the manufacturer not only neglected important information about managing discontinuation syndrome, but also explicitly advised against opening capsules, a practice required to gradually taper dosage.

HIV-associated lipodystrophy, also known as lipodystrophy in HIV-infected patients (LD-HIV), is a condition characterized by loss of subcutaneous fat associated with infection with HIV.

Apitoxins are under preliminary research for their potential biological effects, such as in cancer.

In general, epithelioid sarcoma is a slow-growing and relatively painless tumor, often resulting in a lengthy period of time between presentation and diagnosis. Due to its ambiguity, it is often misdiagnosed, mistaken as a persistent wart or cyst. It most commonly presents itself in the distal limbs (fingers, hands, forearms, or feet) as a small, soft mass or a series of bumps. It is most often described as a firm-to-hard palpable mass, either in the deep soft tissue or in the dermis. Often, superficial lesions will ulcerate causing a mistaken diagnosis of a poorly healing traumatic wound or wart. About 13% of patients will present with multifocal tumors, and about 13% of patients will present with metastatic disease.

Epithelioid sarcoma is a rare soft tissue sarcoma arising from Mesenchyme tissue and characterized by epithelioid-like features. It accounts for less than 1% of all soft tissue sarcomas. It was first clearly characterized by F.M. Enzinger in 1970. It commonly presents itself in the distal limbs (fingers, hands, forearms, or feet) of young adults as a small, soft mass or a series of bumps. A proximal version has also been described, frequently occurring in the upper extremities. Rare cases have been reported in the pelvis, vulva, penis, and spine.

Histologically, epithelioid sarcoma forms nodules with central necrosis surrounded by bland, polygonal cells with eosinophilic cytoplasm and peripheral spindling. Epithelioid sarcomas typically express vimentin, cytokeratins, epithelial membrane antigen, and CD34, whereas they are usually negative for S100, desmin, and FLI-1. They typically stain positive for CA125.

Epithelioid sarcoma most commonly strikes young adults, yet no age group is immune. The disease has a tendency to develop local recurrences and metastasis thereafter to regional lymph nodes, lung, bone, brain, and other locations, including the scalp. Generally speaking, epithelioid sarcoma has a high rate of relapse after initial treatment and tends to recur locally (at or near the original tumor site). Epithelioid sarcoma also demonstrates lymphatic spread (in 22-48% of cases), and metastasis (in 21-63% of cases). These events, as well as advanced stage (progression) and grade (aggressiveness), are predictive of an overall worse outcome. The overall five-year survival rate for epithelioid sarcoma is anywhere from 25 to 78%. Importantly, the 10-year and 15-year survival rate drops off significantly. Associated with a more positive outcome are younger age, female vs. male sex, distal vs. proximal location, smaller tumor size, and negative margins upon tumor resection.

CNE is a necrotizing inflammation of the small bowel (especially the jejunum but also the ileum). Clinical results may vary from mild diarrhea to a life-threatening sequence of severe abdominal pain, vomiting, bloody stool, ulceration of the small intestine with leakage (perforation) into the peritoneal cavity and possible death within a single day due to peritonitis. Many patients exhibit meteorism. Treatment involves suppressing the toxin-producing organisms with antibiotics such as penicillin G or metronidazole. About half of seriously ill patients require surgery for perforation, persistent intestinal obstruction, or failure to respond to the antibiotics. An investigational toxoid vaccine has been used successfully in some developing countries but is not available outside of research.

The myeloproliferative neoplasms (MPNs), previously myeloproliferative diseases (MPDs), are a group of diseases of the bone marrow in which excess cells are produced. They are related to, and may evolve into, myelodysplastic syndrome and acute myeloid leukemia, although the myeloproliferative diseases on the whole have a much better prognosis than these conditions. The concept of myeloproliferative disease was first proposed in 1951 by the hematologist William Dameshek. In the most recent World Health Organization classification of hematologic malignancies, this group of diseases was renamed from "myeloproliferative diseases" to "myeloproliferative neoplasms". This reflects the underlying clonal genetic changes that are a salient feature of this group of disease.

The increased numbers of blood cells may not cause any symptoms, but a number of medical problems or symptoms may occur. The risk of thrombosis is increased in some types of MPN.

An androgen-dependent condition, disease, disorder, or syndrome, is a medical condition that is, in part or full, dependent on, or is sensitive to, the presence of androgenic activity in the body.

Known androgen-dependent conditions include acne, seborrhea, androgenic alopecia, hirsutism, hidradenitis suppurativa, precocious puberty in boys, hypersexuality, paraphilias, benign prostatic hyperplasia (BPH), prostate cancer, and hyperandrogenism in women such as in polycystic ovary syndrome (PCOS).

Such conditions may be treated with drugs with antiandrogen actions, including androgen receptor antagonists such as cyproterone acetate, spironolactone, and bicalutamide, 5α-reductase inhibitors such as finasteride and dutasteride, CYP17A1 inhibitors such as abiraterone acetate, gonadotropin-releasing hormone (GnRH) analogues such as leuprolide and cetrorelix, and/or other antigonadotropins such as megestrol acetate and medroxyprogesterone acetate.