Ten Major Categories of Adverse Drug Reactions
Recently, while studying Pharmacology, I have learned a great deal about drugs, including their adverse reactions. The textbook covers adverse reactions quite comprehensively. I have organized them into notes and now write an article based on those notes to introduce adverse drug reactions, for readers' reference.
Drugs are a double-edged sword. While exerting a therapeutic effect, they may also produce responses unrelated to the purpose of treatment that bring discomfort or suffering to the patient; these are collectively called adverse drug reactions. Understanding the types and characteristics of adverse reactions is an important prerequisite for safe and rational medication. The following explains the adverse drug reactions one by one.
I. Side effects
Side effects (also called secondary effects) refer to responses unrelated to the therapeutic purpose that appear at therapeutic doses. Side effects are inherent to the drug itself, mostly mild and predictable. They arise from the drug's low selectivity and broad range of action; they are generally mild and reversible, but may cause discomfort, such as drowsiness after taking an antihistamine.
II. Toxic reactions
Toxic reactions refer to clearly harmful responses that occur when the dose is too high, the duration of medication is too long, or the body is overly sensitive to the drug. Toxic reactions are generally predictable and should be avoided as far as possible.
Toxic reactions are divided into acute toxicity and chronic toxicity.
Acute toxicity refers to toxic reactions that arise rapidly from excessive dosage, mostly damaging circulatory, respiratory, and nervous system function. Chronic toxicity refers to toxic reactions that gradually develop from prolonged accumulation in the body, mostly damaging liver, kidney, bone marrow, or endocrine organ/system function.
In addition, some drugs can cause the "three-rogen reactions" (carcinogenic, teratogenic, and mutagenic effects), which also fall under chronic toxicity and require great vigilance.
In clinical prescribing, one should master drug dosage and dosing intervals to prevent toxic reactions caused by overmedication.
III. Allergic reactions
Allergic reactions, also called hypersensitivity reactions, are pathological immune responses that a sensitized body produces against certain drugs. Their nature is unrelated to the drug's primary effect or dose; they are not easily predictable and vary widely in severity, typically manifesting as drug fever, rash, urticaria, angioedema, asthma, and so on; severe cases may develop anaphylactic shock and even death. The allergen may be the drug itself or its metabolites, or excipients or impurities in the preparation.
Before medication, ask about allergy history and perform skin tests. Those with a positive skin test should not use the drug, but a negative skin test does not guarantee that an allergic reaction will not occur. Even patients with a negative skin test should be observed for half an hour after medication; this interval is derived from large amounts of clinical data.
IV. Idiosyncratic reactions
Idiosyncratic reactions refer to special responses that a small number of patients with congenital genetic abnormalities make to certain drugs. Their nature may differ from that in ordinary people, but is basically consistent with the drug's inherent action, and severity is dose-related. For example, patients with favism (G6PD deficiency) may develop hemolytic anemia when given sulfonamides; the underlying cause is a deficiency of glucose-6-phosphate dehydrogenase.
V. Residual effects
Residual effects refer to pharmacological effects that persist after the blood concentration has fallen below the minimum effective concentration (threshold concentration) following discontinuation of the drug, such as the "hangover" phenomenon of dizziness, drowsiness, and fatigue the next morning after taking barbiturates.
VI. Secondary reactions
Secondary reactions refer to adverse consequences that occur while a drug is exerting its therapeutic effect. For example, long-term use of broad-spectrum antibiotics disrupts the symbiotic state of normal intestinal flora, causing secondary (superimposed) infection.
VII. Withdrawal reactions
Withdrawal reactions refer to the phenomenon in which, after long-term use of a drug and abrupt discontinuation, the symptoms of the original disease worsen or intensify; this is also called the rebound phenomenon. For example, abruptly stopping certain antihypertensive drugs after long-term use can cause blood pressure to rise markedly, even exceeding the pre-medication level. Withdrawal reactions also include discontinuation symptoms—a series of uncomfortable responses in which the original symptoms do not worsen.
VIII. Tolerance and high sensitivity
Tolerance refers to a reduction in the body's sensitivity to a drug after repeated dosing, such that the dose must be increased to achieve the original effect. Drugs prone to inducing tolerance include barbiturates, opioids, nitrates, and insulin.
Tolerance is divided into innate tolerance and acquired tolerance: tolerance present from the first dose is innate; tolerance appearing after repeated dosing is acquired.
High sensitivity is the opposite of tolerance: it refers to the small number of people who are extremely sensitive to a drug. For example, if most people need one tablet of diazepam to achieve an effect, a highly sensitive person may need only half a tablet.
IX. Drug resistance
Drug resistance (also called antimicrobial resistance) refers to the pathogen developing resistance to a drug, so that the drug can no longer exert a therapeutic effect or its efficacy is diminished. Note the distinction: tolerance means "the person tolerates the drug," whereas resistance means "the pathogen (such as bacteria or cancer cells) is no longer sensitive to the drug"; the two are fundamentally different. Resistance that develops after repeated dosing is also called acquired resistance.
X. Dependence
Dependence refers to the subjective and objective need for continued medication that a patient develops after long-term use of certain drugs. It is divided into psychological dependence (emotional dependence) and physical dependence (physiological dependence). Physical dependence is stronger than psychological dependence; those physically dependent on a drug require it to maintain physiological function, and abrupt discontinuation can cause pronounced withdrawal symptoms.
Psychological dependence is 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 late-stage cancer pain, although analgesics may produce some degree of dependence, without them the patient's suffering is unbearable. In such cases, pain relief and improvement of the patient's quality of life should take priority.
Adverse reactions to Chinese medicine must likewise not be ignored. Many people mistakenly believe that "Chinese medicine is natural and non-toxic"; in fact, Chinese medicine also carries the risk of adverse reactions. Once in the body, Chinese herbs likewise act through a series of biochemical reactions; all ten categories of adverse reactions above exist for Chinese medicine, though their intensity may not be as obvious as with certain Western drugs.
I once compiled a 13,000-odd-word article, "A Complete Guide to the Toxicity of Commonly Used Chinese Herbs and Detoxification Methods After Herbal Poisoning." Those interested can click the link to read it; I will not repeat it here.
What can ordinary people do to avoid adverse drug reactions as much as possible? I suggest the following when taking medicine:
- 1. Follow the doctor's instructions; do not adjust the dose or prolong the course on your own; 2. Read the package insert and pay attention to adverse reactions and contraindications; 3. Proactively disclose allergy history and medication history when seeking care; 4. Avoid abusing antibiotics, sedatives, and analgesics; 5. Combining Chinese and Western medicines must be done under the guidance of a doctor or pharmacist; 6. Long-term users should regularly check liver and kidney function and other indices.
In short, adverse drug reactions are numerous: there are predictable toxic reactions and unpredictable allergic reactions. Whether with chemical drugs or Chinese medicine, we should uphold the principle of "safe, effective, and rational" use. Only by fully understanding drug characteristics and using medicine scientifically can we realize the therapeutic value of drugs and minimize harm.