Drug allergy and drug intolerance: What is the difference?
Any medication can cause unwanted effects such as nausea, dizziness, an upset stomach or a rash. Some of these reactions are allergies, caused by the immune system. Others are intolerances: side effects that happen without an immune response, often because the drug builds up in the body or reacts with other medications. Inactive ingredients in pills, such as lactose or dyes, can cause either type. Knowing the difference matters, because a true allergy usually means avoiding the drug, while an intolerance can often be solved with a different dose, drug or brand.
What is a drug intolerance?
A drug intolerance is a side effect that does not involve the immune system. Common causes include:
- Slow drug clearance. Reduced liver or kidney function, or inherited variations in liver enzyme genes such as CYP2D6, CYP2C19 and CYP2C9, can make a drug build up in the body. Side effects then grow with the dose and over days or weeks of treatment.
- Drug interactions. One medication can block or speed up the enzyme that clears another. For example, some antidepressants block CYP2D6 and can make codeine ineffective or raise the levels of other drugs. Read more in Pain and depression drugs not working?
- Inactive ingredients. Sugars such as lactose can cause stomach upset in sensitive people (see below).
Genetic differences in drug metabolism are well documented: the US Food and Drug Administration lists hundreds of drug–gene pairs in its table of pharmacogenomic biomarkers in drug labeling. People who are poor metabolizers for a drug, or who take several interacting medications, are at the highest risk.
What is a drug allergy?
A drug allergy is an immune reaction to an active or inactive ingredient: the body mistakes something in the medication for a threat. There are two main types:
- Immediate reactions, usually within minutes to an hour, are mostly caused by IgE antibodies. They include hives, swelling, wheezing and, in the most severe cases, anaphylaxis: a life-threatening reaction with swelling, trouble breathing and a sudden drop in blood pressure. Penicillin is a common example.
- Delayed reactions appear days to weeks after starting a drug and are driven by immune T cells. Most are mild rashes, but some are severe, such as Stevens-Johnson syndrome or a reaction called DRESS (drug reaction with eosinophilia and systemic symptoms).
An allergy can be triggered by a very small amount of the drug, and the reaction can be more severe if you take the drug again.
Which severe drug allergies are linked to genes?
Stevens-Johnson syndrome (SJS) and its most severe form, toxic epidermal necrolysis (TEN), are rare but dangerous reactions that cause blistering and peeling of the skin and mucous membranes. Common triggers include allopurinol (for gout), the antiepileptics carbamazepine, oxcarbazepine, phenytoin and lamotrigine, sulfa drugs such as sulfamethoxazole, and some anti-inflammatory drugs. In a study of patients treated at Vancouver General Hospital, allopurinol was the most common trigger, and Asian and Indigenous patients had higher death rates than white patients.
For some drugs, specific variations in immune system (HLA) genes greatly increase the risk, and CPIC guidelines advise avoiding the drug in carriers:
| Drug | Gene variant | Reaction |
|---|---|---|
| Carbamazepine, oxcarbazepine | HLA-B*15:02 (more common in people of Southeast Asian ancestry); HLA-A*31:01 (several populations) for carbamazepine | SJS/TEN, DRESS and other skin reactions |
| Phenytoin | HLA-B*15:02; CYP2C9 variants also raise phenytoin levels | SJS/TEN |
| Allopurinol | HLA-B*58:01 (more common in people of Han Chinese, Korean, Thai and African ancestry) | SJS/TEN, DRESS |
| Abacavir (HIV) | HLA-B*57:01 | A different severe hypersensitivity reaction |
Testing for HLA-B*57:01 before abacavir is standard care, and drug labels recommend HLA-B*15:02 testing before carbamazepine in people of at-risk ancestry. Ask your doctor whether HLA testing is right for you before starting one of these drugs. HLA testing does not catch every case, however: in a 2025 US study, more than one-third of people who developed severe skin reactions to allopurinol did not carry HLA-B*58:01, the variant usually tested for (45% of Black patients). Some of them carried other HLA variants, such as HLA-A*34:02, that a single-variant test would miss. Other risk factors, such as reduced kidney function and a high starting dose, also matter, which is why allopurinol is usually started at a low dose and increased gradually.
Current HLA tests also have limits. Most look for only one or a few variants, a positive result does not mean a reaction will happen, and the predictive value differs between drugs and populations, so HLA genotyping is a supportive rather than a definitive tool. After a delayed reaction such as DRESS, especially when several drugs were started at the same time, patch testing done by an allergy specialist months after recovery can help identify which drug caused it, and blood tests of immune cell reactions are available in some specialized centres.
For more on severe reactions such as Stevens-Johnson syndrome and DRESS, and how the responsible drug is identified, read Medication hypersensitivity: genetics and drug toxicity.
Can inactive ingredients cause reactions?
Yes. A study of more than 42,000 pills and capsules found that:
- An average pill contains about 9 inactive ingredients, which make up most of its weight.
- 92.8% of oral medications contain at least one ingredient that has been linked to allergic reactions.
- 45% contain lactose and 55% contain at least one FODMAP sugar, which can trouble people with irritable bowel syndrome (IBS).
- 33% contain a dye that has been linked to allergic reactions, such as tartrazine.
- All progesterone capsules and 62.5% of valproic acid capsules in the US database contained peanut oil.
The amount of lactose in most pills is too small to cause symptoms in most people with lactose intolerance, but it can add up for people with severe intolerance who take several medications. The same drug from different manufacturers can contain different inactive ingredients, which may be one reason some people react to one generic but not another. If you have a food allergy, celiac disease or lactose intolerance, ask your pharmacist to check the inactive ingredients of your medications.
How can you tell a drug allergy from an intolerance?
| Feature | Drug allergy | Drug intolerance |
|---|---|---|
| Cause | Immune system (IgE antibodies or T cells) | Drug build-up from slow metabolism or clearance, drug interactions, or inactive ingredients |
| Dose | Even a tiny dose can trigger it | Usually worse at higher doses |
| Timing | Minutes to an hour (immediate) or days to weeks (delayed) | Often builds over days to weeks, or soon after an interacting drug is added |
| Typical signs | Hives, swelling, wheezing, rash, fever; rarely blistering skin or anaphylaxis | Nausea, dizziness, drowsiness, headache, stomach upset; the expected side effects of the drug, but stronger |
| What helps | Avoiding the drug (or the ingredient, if the allergy is to an inactive ingredient) | A lower dose, a different drug, removing an interacting drug, or a brand without the problem ingredient |
Always report a suspected drug allergy to your health care provider. Seek emergency care for swelling of the face or throat, trouble breathing, or blistering or peeling skin.
Can a DNA test help?
Pharmacogenetic tests such as Pillcheck show how your liver enzymes process many medications, which helps explain side effects caused by slow or fast metabolism and drug interactions. A Pillcheck pharmacist reviews your results together with all your medications and recommends safer options and doses. If you or a family member seem "sensitive" to many medications, a pharmacogenetic test can be a useful personal prescribing guide. For an example, read Tylenol 3 side effects: why codeine doesn't work for everyone.
Frequently asked questions
How do I know if I am allergic to a medication?
Hives, swelling, wheezing or a rash after taking a drug suggest an allergy, especially if a small dose triggered it. Your doctor may refer you to an allergist for testing.
Is a side effect the same as an allergy?
No. Most side effects, such as nausea or dizziness, are not allergic. They are often caused by high drug levels from slow metabolism or drug interactions and can be managed by changing the dose or the drug.
Can genes cause drug allergies?
Some severe allergic reactions are strongly linked to HLA genes, such as HLA-B*15:02 with carbamazepine and HLA-B*58:01 with allopurinol. Genes such as CYP2D6 and CYP2C19 cause intolerance rather than allergy.
Why do I react to a generic but not the brand-name drug?
Different manufacturers use different inactive ingredients, such as lactose, dyes or oils. Your pharmacist can help find a version without the ingredient that bothers you.
Can a pharmacogenetic test tell me if I have a drug allergy?
Not for most allergies, which need an allergist's assessment. A pharmacogenetic test shows how you metabolize medications, which explains many intolerances, and HLA testing can predict some severe reactions.
References
Reker D, Blum SM, Steiger C, et al. "Inactive" ingredients in oral medications. Sci Transl Med. 2019;11(483):eaau6753.
Miliszewski MA, Kirchhof MG, Sikora S, et al. Stevens-Johnson Syndrome and Toxic Epidermal Necrolysis: An Analysis of Triggers and Implications for Improving Prevention. Am J Med. 2016;129(11):1221-1225.
Phillips EJ, Sukasem C, Whirl-Carrillo M, et al. Clinical Pharmacogenetics Implementation Consortium Guideline for HLA Genotype and Use of Carbamazepine and Oxcarbazepine: 2017 Update. Clin Pharmacol Ther. 2018;103(4):574-581.
Karnes JH, Rettie AE, Somogyi AA, et al. Clinical Pharmacogenetics Implementation Consortium (CPIC) Guideline for CYP2C9 and HLA-B Genotypes and Phenytoin Dosing: 2020 Update. Clin Pharmacol Ther. 2021;109(2):302-309.
Saito Y, Stamp LK, Caudle KE, et al. Clinical Pharmacogenetics Implementation Consortium (CPIC) guidelines for human leukocyte antigen B (HLA-B) genotype and allopurinol dosing: 2015 update. Clin Pharmacol Ther. 2016;99(1):36-37.
Campbell CN, Krantz MS, Yu A, Phillips EJ, et al. HLA-B*58:01 and Risk of Allopurinol-Induced Severe Cutaneous Adverse Reactions in the US. JAMA Dermatol. 2025;161(12):1258-1263.
Ünal D, Yeğit OO, Demir S, et al. Utility of Patch Testing, LTT, and HLA Genotyping in Identifying Culprit Drugs and Diagnosing Multiple Drug Hypersensitivity in Definite DRESS: A 10-Year Cohort Study. Clin Transl Allergy. 2026;16(4):e70172.
Martin MA, Hoffman JM, Freimuth RR, et al. Clinical Pharmacogenetics Implementation Consortium Guidelines for HLA-B Genotype and Abacavir Dosing: 2014 update. Clin Pharmacol Ther. 2014;95(5):499-500.
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