People administer drugs when they find that it has some therapeutic aspects. They take drugs to maintain or cure an ailment. They can have either desirable (therapeutic) or undesirable effects (allergy) on the person taking them. Scientists classify ADR into five types (A, B, C, D, E). Adverse drug reactions are unintended and occur at high doses of drugs used for prophylaxis, diagnosis, or therapy. The reactions can be fatal or moderate depending on the individual and drug’s pharmacology. Some examples include hypoglycemia due to insulin injection and bradycardia due to beta-adrenoceptor. Some types of drugs can have unpredictable effects and often have a high rate of mortality. An example includes the suppression of bone marrow by chloramphenicol. Adverse events (AEs) are unpredicted medical occurrences in the patient or clinical trial subject after administration of medicine.

Adverse drug reactions
Generally, when pharmaceutical scientists develop drugs, they expect the drug to act at the specific target site and not in other body systems. However, if a pharmaceutical drug shows its influence in sites other than intended, it can have harmful effects.

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When a person takes antihistamines such as cetirizine for cold or allergy, it brings relief to the person. However, after a period, it can make the person feel drowsy. Therefore, any medication produced to provide therapeutic effects can also produce unwanted and undesirable effects. An adverse drug reaction is an undesired reaction due to the administration of a medicinal product. It may occur with the administration of a single dose or prolonged administration of a drug. It can also arise from the reaction of two different drugs in the body. For example, nonsteroidal anti-inflammatory drugs (NSAIDs) are used as pain killers and decrease inflammation and fever. People who regularly use NSAIDs often encounter stomach irritation and bleeding. These unwanted effects arise because apart from acting on the disease site, NSAIDs influence the production of prostaglandins by decreasing their levels.

Classification of ADR
Adverse drug reactions can be classified as follows:

Type A (Augmented drug reaction): These reactions occur when a person overdoses on certain drugs. It can include side effects, secondary effects, and toxic effects. The reactions depend on the pharmacological profile, drug profile, and dose of the drug. The adverse reactions from these drugs can be brought under control if they are below a limit. For example, taking cough syrup or antihistamines makes a person feel drowsy after some time. Since people are already aware of these reactions, they are called side effects. When the dose of a drug crosses the maximum effective range, it can result in toxic effects.

Type B (Bizarre): These drug reactions are rare and can cause allergic reactions and idiosyncratic reactions. If the drug taken by a person initiates a hypersensitivity response from the immune system, it can result in allergic reactions. They can suffer from swelling, itching, and difficulty breathing and have skin reactions. Idiosyncratic reactions are rare since they function well in the population but might harm a particular individual. These individuals usually have altered genetic sequences leading to the harmful effect. For example, a person might develop deep vein thrombosis by taking oral contraceptives.

The various types of bizarre reactions are as follows:

Type I (Anaphylaxis): The drugs that can result in this reaction are penicillin and lidocaine.

Type II (Cytolytic reaction): For example, development of anemia when a person takes phenytoin. Carbamazepine consumption may also cause a cytolytic reaction.

Type III (Arthus reaction): Sulfonamide triggers this reaction.

Type IV (Delayed hypersensitivity): An example of this allergic reaction is allergic contact dermatitis caused by penicillin.

Type C (Chronic use): These reactions occur when a person takes the drug for extended periods. An example of this effect is analgesic nephropathy.

Type D (Delayed): These drugs show their response after the intended period. Examples include osteonecrosis of the jaw with bisphosphonates, leucopenia due to lomustine, and teratogenic effects.

Type E (End of use): These effects are apparent in people who suddenly withdraw from a drug and develop harmful effects. Examples include angina after atenolol, withdrawal after opioids and alcohol, and so on.