The Ten Major Categories of Adverse Drug Reactions

The Ten Major Categories of Adverse Drug Reactions

Recently, while studying Pharmacology, I came across a wealth of knowledge about medications, including adverse drug reactions. The textbook provided a fairly comprehensive overview, so I organized my notes and have written this article specifically to introduce the adverse reactions of drugs for readers' reference.

Medications are a double-edged sword. While they exert their therapeutic effects, they may also cause reactions that are unrelated to the intended purpose of the medication and bring discomfort or suffering to the patient. These reactions are collectively referred to as adverse drug reactions. Understanding the types and characteristics of adverse drug reactions is an important prerequisite for safe and rational drug use. Below is a detailed explanation of each category.

1. Side Effects

Side effects refer to effects that occur at therapeutic doses and are unrelated to the intended purpose of the medication. Side effects are inherent to the drug itself; most are mild and predictable. They result from the drug's low selectivity and broad range of action. Generally mild and reversible, they may nonetheless cause discomfort to the patient — for example, drowsiness after taking an antihistamine.

2. Toxic Reactions

Toxic reactions refer to responses that cause significant harm to the body when the dosage is excessively large, the duration of use is too long, or the body has an abnormally high sensitivity to the drug. Toxic reactions are generally predictable and should be avoided as much as possible.

Toxic reactions are divided into two types: acute toxicity and chronic toxicity.

Acute toxicity refers to toxic reactions that occur rapidly due to an excessively large dose, often impairing the functions of the circulatory, respiratory, and nervous systems. Chronic toxicity refers to toxic reactions that develop gradually as a result of long-term drug use leading to excessive accumulation in the body, often impairing the functions of the liver, kidneys, bone marrow, endocrine organs, or organ systems.

In addition, some drugs can cause carcinogenic, teratogenic, and mutagenic effects — collectively known as the "three mutagenic effects" — which also fall under chronic toxicity and require heightened vigilance.

When administering drugs clinically, attention should be paid to proper dosage and dosing intervals to prevent toxic reactions caused by overmedication.

3. Allergic Reactions

Allergic reactions, also known as hypersensitivity reactions, refer to pathological immune responses produced by a sensitized body against certain drugs. Their nature is unrelated to the drug's original pharmacological effects or dosage, and they are difficult to predict. The severity varies widely, typically manifesting as drug fever, skin rashes, urticaria, angioedema, asthma, and other symptoms. In severe cases, anaphylactic shock or even death may occur. The allergen responsible may be the drug itself, its metabolites, or the excipients and impurities present in the formulation.

Before administration, patients should be asked about their allergy history and given a skin test. Those with a positive skin test are contraindicated from using the drug, and even those with a negative skin test are not guaranteed to be free from allergic reactions. Patients with a negative skin test should still be observed for half an hour after administration — this duration is based on extensive clinical statistical data.

4. Idiosyncratic Reactions

Idiosyncratic reactions refer to unusual responses to certain drugs that occur in a small number of patients with congenital genetic abnormalities. The nature of the reaction may differ from that in the general population but is generally consistent with the drug's inherent pharmacological action, and the severity is dose-related. For example, patients with favism (glucose-6-phosphate dehydrogenase deficiency) may develop hemolytic anemia when administered sulfonamide drugs, the underlying cause being a deficiency of glucose-6-phosphate dehydrogenase.

5. Residual Effects (After-Effects)

Residual effects refer to pharmacological effects that persist after the drug has been discontinued and its blood concentration has fallen below the minimum effective concentration (threshold concentration). An example is the "hangover" phenomenon — dizziness, drowsiness, and fatigue the morning after taking barbiturates.

6. Secondary Reactions

Secondary reactions refer to adverse consequences that arise as a result of the drug exerting its therapeutic effect. For example, long-term use of broad-spectrum antibiotics can disrupt the symbiotic balance of normal intestinal flora, leading to secondary (superinfection).

7. Withdrawal Reactions

Withdrawal reactions refer to the phenomenon in which symptoms of the original disease are exacerbated or intensified after abruptly discontinuing a drug that has been used over a long period. This is also known as the rebound phenomenon. For example, long-term use of certain antihypertensive drugs, followed by sudden discontinuation, can cause blood pressure to rise sharply, even exceeding pre-treatment levels. In addition, withdrawal reactions also include withdrawal symptoms — a series of uncomfortable reactions, though without worsening of the original symptoms.

8. Tolerance and Hypersensitivity

Tolerance refers to a decreased sensitivity of the body to a drug after repeated use, requiring an increased dose to achieve the original effect. Drugs that are prone to inducing tolerance include barbiturates, opioids, nitrates, and insulin.

Tolerance is divided into congenital tolerance and acquired tolerance: tolerance that is evident from the very first dose is called congenital tolerance, while tolerance that develops only after multiple doses is called acquired tolerance.

Hypersensitivity is the opposite of tolerance. It refers to a condition in which a small number of individuals are highly sensitive to a drug. For example, while most people require one tablet of diazepam to achieve a therapeutic effect, a few individuals may respond to just half a tablet — such individuals are considered hypersensitive.

9. Drug Resistance

Drug resistance, also known as antimicrobial resistance (or simply drug resistance), refers to the phenomenon in which a pathogen develops resistance to a drug, rendering the drug unable to exert its therapeutic effect or significantly reducing its efficacy. It is important to distinguish this from tolerance: tolerance means "the body becomes tolerant to the drug," whereas drug resistance means "the pathogen (such as bacteria or cancer cells) is no longer sensitive to the drug." The two are fundamentally different in nature. Drug resistance that develops after repeated exposure is referred to as acquired resistance.

10. Drug Dependence

Drug dependence refers to the phenomenon in which, after prolonged use of certain drugs, the patient develops a subjective and objective need for continuous medication. It can be divided into psychological dependence and physical dependence. Physical dependence is more severe than psychological dependence; individuals who develop physical dependence require the drug to maintain normal physiological function, and abrupt discontinuation can cause pronounced withdrawal symptoms.

Psychological dependence refers to a special mental effect produced by the drug's action on the central nervous system, manifesting as an intense craving for the drug and compulsive drug-seeking behavior.

It should be noted that for patients with advanced cancer pain, although analgesics can produce a certain degree of dependence, not using them would subject the patient to unbearable suffering. In such cases, pain relief and improvement of the patient's quality of life should take priority.

Adverse Reactions of Traditional Chinese Medicine

Adverse reactions of traditional Chinese medicine (TCM) should not be overlooked either. Many people mistakenly believe that "TCM is natural and therefore non-toxic," but this is far from the truth. TCM also carries risks of adverse reactions. Once introduced into the body, TCM also exerts its effects through a series of biochemical reactions. All ten categories of adverse reactions described above can occur with TCM as well — it is only that the intensity may not be as pronounced as with some Western medicines.

The author has previously compiled a comprehensive article of over 13,000 words titled "A Complete Guide to the Toxicity of Commonly Used Chinese Medicinal Herbs and Antidotes for Chinese Medicine Poisoning." Those interested may follow the link to read that article, so it will not be elaborated upon here.

What Ordinary People Can Do to Minimize Adverse Drug Reactions

What can ordinary people do to minimize adverse drug reactions? The author recommends paying attention to the following points when using medications:

1. Take medications as prescribed by your doctor; do not adjust the dose or extend the course of treatment on your own.
2. Read the drug information leaflet and pay attention to adverse reactions and contraindications.
3. Proactively inform your doctor of your allergy history and medication history during consultations.
4. Avoid the misuse of antibiotics, sedatives, analgesics, and similar medications.
5. The combined use of Chinese and Western medicines should be conducted under the guidance of a physician or pharmacist.
6. Patients on long-term medication should undergo regular monitoring of liver and kidney function and other relevant indicators.

Summary

In summary, adverse drug reactions come in many forms — some, like toxic reactions, are predictable, while others, like allergic reactions, are difficult to foresee. Whether dealing with chemical drugs or traditional Chinese medicine, the principle of "safe, effective, and rational" drug use should always be upheld. Only by fully understanding the characteristics of medications and using them scientifically can we truly realize their therapeutic value while minimizing harm to the greatest extent possible.