OVERVIEW

What is drug allergy?

Drug allergy refers to allergic reactions that occur in a small number of patients with special constitutions when using normal or below-normal doses of medication. Mild cases may involve skin itching, rashes, or fever, while severe cases can lead to anaphylactic shock, even life-threatening situations.

SYMPTOMS

How long after medication do drug allergy symptoms appear?

Drug allergies typically occur 4–20 days (average 7–8 days) after repeated exposure to the same medication. If a drug allergy has already occurred (during initial exposure, symptoms may not appear, but the body prepares for a severe reaction), reusing the same drug can trigger an immediate allergic reaction, with symptoms appearing within 72 hours at the latest.

For example, when penicillin is first used, the immune system must first recognize the drug, then activate immune cells to produce specific antibodies or sensitized lymphocytes before an allergic reaction occurs. This sensitization process takes at least 3 days. However, upon reusing penicillin, since the body already has specific antibodies, an allergic reaction can occur immediately—ranging from mild hives to severe anaphylactic shock, which can be life-threatening without prompt treatment.

What are the symptoms and manifestations of a drug allergy?

Drug allergies often cause systemic reactions, such as anaphylactic shock. Some manifest as various skin rashes, called drug-induced dermatitis or drug eruptions; others only affect the blood or internal organs; some involve both rashes and internal organ damage.

What internal organ damage can drug allergies cause?

Beyond visible rashes, drug allergies can also cause "internal injuries." Common issues include swollen lymph nodes, enlarged liver or spleen, impaired liver or kidney function, asthma, and interstitial pneumonia.

What are the skin manifestations of drug-induced dermatitis from allergies?

Drug allergy skin reactions vary widely and can be classified into over a dozen clinical types.

Early signs often include redness, hives (raised patches), or papules (small red bumps), followed by bleeding spots, blisters, or pustules, then necrosis, ulcers, and scabbing. Finally, peeling and hyperpigmentation or hypopigmentation may occur.

Severe cases may involve red eyes, oral mucosa erosion, or full-body skin shedding resembling severe burns (exfoliative dermatitis), which can be life-threatening.

Besides rashes, what other symptoms can drug allergies cause?

In addition to rashes, drug allergies can trigger sneezing, runny nose, asthma attacks, or even systemic edema (e.g., throat swelling leading to rapid breathing difficulty), low blood pressure, shock, and other life-threatening conditions.

CAUSES

What Causes Drug Allergies?

The occurrence of any event is the result of the interaction between internal and external factors, with internal factors being the primary cause. The external factor is the drug, while the internal factor is your allergic constitution. For example, the same drug, such as penicillin, may cause severe allergic reactions in some individuals, while most people experience no discomfort after use. This difference is due to variations in constitution.

Which Drugs Commonly Cause Allergies?

Strictly speaking, any drug has the potential to cause allergies. However, clinically, the following categories of drugs are commonly associated with allergic reactions:

• Antibiotics: Such as penicillins, sulfonamides, and cephalosporins.
• Antipyretic and Analgesic Drugs: Such as aspirin, indomethacin, ibuprofen, aminopyrine, metamizole, and compound analgesics.
• Sedatives and Anticonvulsants: Such as phenobarbital, phenytoin, and chlorpromazine.
• Biological Products: Such as blood transfusions, human serum albumin, plasma, and tetanus antitoxin.

Do Drug Allergies Only Occur with Oral or Injected Medications?

No.

People often assume that drug allergies only occur with oral or injected medications, but this is not the case. Allergic reactions can happen as long as the drug enters the body. For example, inhaling medication during nebulization, topical application through the skin, or rectal suppositories can all trigger drug allergies.

Can Drug Allergies Occur Without "Taking Medication"?

Possibly.

Sometimes, allergic reactions occur even without taking oral or injected medications or using drugs through other routes. This is because certain foods, especially animal-based products, may contain drug residues. For instance, if antibiotics are illegally used in animal farming, traces may remain in meat products. Consuming such foods can unknowingly introduce these drugs into the body, leading to allergies.

What Is the Mechanism of Drug Allergies?

This is a highly specialized topic that may be difficult for non-medical professionals to understand. Allergic reactions are typically classified into four types:

• Type I (Immediate Hypersensitivity): Occurs shortly after drug administration and is the most common type. It is characterized by elevated IgE and eosinophils, manifesting as anaphylactic shock, urticaria, or drug fever.
• Type II: Features IgG elevation without significant complement changes, leading to blood-related damage such as acute hemolytic anemia, leukopenia, or thrombocytopenia.
• Type III: Involves IgG elevation with complement depletion and organ damage, presenting as allergic kidney, liver, or vascular-nerve injuries.
• Type IV (Delayed Hypersensitivity): Occurs more than three days after exposure, characterized by excessive hypersensitive T-cell activity and severe skin damage, such as toxic epidermal necrolysis or severe erythema multiforme.

Is Drug Allergy Related to Dosage?

No.

Whether a drug causes an allergic reaction is unrelated to dosage. For individuals not allergic to a particular drug, even toxic doses will not trigger an allergic response.

Can Sun Exposure Cause Drug Allergies?

Possibly.

Sun exposure can indeed trigger allergies to certain photosensitive drugs, such as quinolones (e.g., norfloxacin, levofloxacin), thiazides (e.g., hydrochlorothiazide), tetracyclines, and contraceptives. Using these drugs while exposed to sunlight may cause allergic reactions, so sun protection is advised.

If a Drug Didn’t Cause an Allergy Before, Can It Cause One Later?

Yes.

Allergic reactions depend on the body's physiological state at the time. If immunity is weakened or a viral infection is present, previously non-allergenic drugs may trigger allergies.

If a Drug Caused an Allergy Before, Will It Always Cause One Later?

In most cases, once an allergy occurs, re-exposure will trigger another reaction.

Drugs causing Type II, III, or IV hypersensitivity reactions, or severe Type I reactions (e.g., anaphylactic shock or angioedema), must never be reused, as symptoms may worsen and become life-threatening. However, exceptions exist—if immunity changes (e.g., immune tolerance or severe immunosuppression), allergies may not recur.

Can Skin Tests Reduce Drug Allergies? Why?

Skin tests involve injecting a tiny amount of the drug intradermally. If the person has previously developed allergic antibodies to the drug, an immediate reaction (e.g., wheals or erythema) indicates a positive test, and the drug must be avoided.

Note: Even minimal doses in skin tests can sometimes cause fatal shock. If allergic to a drug, inform the doctor to avoid testing. Follow medical staff instructions and wait in the observation area—do not leave prematurely, as emergency equipment is on standby.

If a Skin Test Is Negative, Will No Allergy Occur?

No.

A 20–30 minute observation after testing only rules out Type I (immediate) reactions. Since allergies can have latent periods, other types may still develop later.

DIAGNOSIS

How to Identify Drug Allergic Reactions?

Drug allergic reactions have the following characteristics:

• There is a certain "latent period," usually 4–20 days, averaging 7–8 days. If sensitized, re-exposure to the same drug often triggers symptoms within 24 hours, averaging 7–8 hours. The shortest onset is within minutes, while the longest does not exceed 72 hours.
• Most cases begin suddenly, though some may have precursor symptoms like chills, discomfort, or fever.
• Except for fixed drug eruptions, rashes typically develop in a generalized and symmetrical pattern (appearing all over the body, bilaterally symmetrical).
• Often accompanied by systemic reactions of varying severity, ranging from mild (barely noticeable) to severe (headache, chills, high fever, etc.).
• Symptoms usually subside after discontinuing the drug—mild cases resolve in about a week, while severe cases generally improve within a month.

What Conditions Should Drug Allergies Be Distinguished From?

Exanthematous drug eruptions should be differentiated from scarlet fever and measles.

Measles patients first develop a fever, which can reach 40°C. After four days of fever, a rash appears at the hairline (the boundary between hair-bearing and non-hair-bearing skin) before spreading to the entire body. The rash is usually non-itchy or mildly itchy, and the fever subsides after the rash emerges. Scarlet fever patients first experience a sore throat accompanied by fever, with the rash appearing two days after the fever begins.

What Tests Can Be Done for Drug Allergies?

Despite rapid advancements in science and technology, medical capabilities remain limited. Currently, there are no simple and definitive clinical tests to identify specific drug allergies.

For laboratory tests, skin scratch and intradermal tests are often used to assess sensitivity to penicillin or iodides, which can help prevent anaphylactic shock but have limited value in preventing drug allergies.

In vitro tests, such as lymphocyte transformation tests and radioallergosorbent tests, have been used to detect allergens. However, they are only reliable for certain drugs and are currently confined to experimental research in a few laboratories, not yet applied in clinical practice.

TREATMENT

Which department should I visit for drug allergies?

Rheumatology and Immunology Department, Emergency Department. If it's just simple skin symptoms, you can also visit the Dermatology Department.

What should I do if a drug allergy occurs?

If a drug allergy occurs, the first and most important step is to immediately stop using the suspected allergenic drug. The second step is to seek medical treatment at a hospital right away. Do not buy and take medication on your own to avoid delaying treatment and missing the optimal treatment window.

Can drug allergies heal on their own without treatment?

A very small number of mild cases may heal on their own if the allergenic drug is stopped early. However, most drug allergic reactions require hospital treatment to fully recover.

How is drug allergy treated?

For oral drug allergies, severe reactions are less common, so antihistamines or hormones are often prescribed. For non-oral drug allergies, such as intramuscular or intravenous injections, severe reactions are more likely, and epinephrine injections should be administered immediately.

Can drug allergies be life-threatening?

Severe drug allergies can be fatal.

Laryngeal edema and anaphylactic shock can lead to immediate death if not treated promptly.

Other types, such as erythroderma (exfoliative dermatitis), toxic epidermal necrolysis, severe erythema multiforme, and drug hypersensitivity syndrome, are all severe drug allergic reactions that are life-threatening and require immediate hospitalization.

Therefore, if a drug allergy occurs, do not self-medicate. Always seek treatment at a proper hospital and take medication under a doctor's guidance. This is not something to DIY.

When will drug allergy symptoms subside?

If the allergen has been removed, the time for symptoms to subside varies because people react differently to allergies, and the amount of drug absorbed also differs.

In terms of presentation, some may show acute or chronic processes. Acute symptoms subside quickly, often within the same day, while chronic symptoms may take up to a week to fully disappear. For severe skin manifestations of Type IV hypersensitivity reactions, it may take 2–4 weeks or even 2–3 months to fully recover.

DIET & LIFESTYLE

What should be paid attention to in diet and daily life after drug allergy?

Allergy is caused by drugs and generally does not affect daily contact or diet. However, if you are usually allergic to certain contact items or foods, or suspect allergies, you should avoid them during this period to prevent worsening the allergic reaction.

PREVENTION

How to Prevent Drug Allergies?

Prevention is key when it comes to drug allergies. If an allergic reaction occurs while taking medication, remember the name of the drug to avoid accidental reuse. Extra caution should be taken when administering drugs to individuals with allergic tendencies. If an allergic reaction occurs, the most important step is to immediately discontinue the medication.

Currently, some countries have implemented genetic screening to prevent certain drug allergies by checking whether patients carry genes associated with drug allergies. It is believed that in the future, drug allergic reactions will become increasingly rare.