Can Antihistamines Like Allegra and Zyrtec Cause Weight Gain?

Antihistamines can be great at relieving allergy symptoms such as sneezing, runny nose, and itchy, watery eyes. While the drugs are generally regarded as safe, they are not without side effects.

One possible side effect—and one that people don't often think about—is the risk of weight gain with over-the-counter antihistamines like Allegra (fexofenadine) and Zyrtec (cetirizine).

This article explores differing views on whether antihistamines cause weight gain and offers some research insights and possible explanations for why weight gain can occur.

Woman unhappy looking at a scale
Tetra Images / Getty Images

What Are Antihistamines?

Antihistamines are medications that are commonly used to treat allergy symptoms due to conditions like:

These drugs work by blocking the actions of histamine, a chemical released by the immune system in response to certain triggers. Histamine is normally released in response to infection, while also helping to regulate blood pressure, digestion, and the sleep-wake cycle.

When histamine is released as part of an allergic response, it can cause itching, rash, breathing problems, and other adverse symptoms. Histamine is the primary cause of allergy symptoms.

Antihistamines work by binding to histamine receptors on the brain, spinal cord, smooth muscles, and other tissues of the body. Without these attachment points, histamine cannot act on the body and cause allergy symptoms.

Can Antihistamines Cause Weight Gain?

Some studies of weight gain and its relationship to antihistamine use find there's a pattern of association, but it's not entirely clear why.

Weight gain may be linked to changes in lymph flow that affect lipids (fats) and digestive absorption. Both obesity and metabolic syndromes (like metabolic dysfunction-associated steatotic liver disease, or MASLD) may arise from the effect of long-term antihistamine use.

These conclusions from a 2020 study, published in the journal Children, also found that prolonged antihistamine use in children was associated with a two-fold increase in body mass index (BMI) per year compared to kids who didn't take antihistamines.

BMI is a dated, flawed measure. It does not take into account factors such as body composition, ethnicity, sex, race, and age. Even though it is a biased measure, BMI is still widely used in the medical community because it’s an inexpensive and quick way to analyze a person’s potential health status and outcomes.

Older antihistamines, such as Benadryl (diphenhydramine), are known to cause significant side effects such as drowsiness.

Newer antihistamines, like Allegra, were seen to be an advance in treatment, alleviating symptoms without the accompanying drowsiness. Even so, newer antihistamines like Allegra and Zyrtec are known to cause side effects like:

According to a landmark 2010 study published in the journal Obesity, people who regularly take antihistamines are more likely to be overweight or obese than those who do not.

With 268 antihistamine users involved in the study, their prolonged use was associated with a roughly 10% increase in weight, body mass index (BMI), and waist circumference. The risk was seen to be greatest with newer-generation drugs like Allegra, Zyrtec, Claritin (loratadine), Clarinex (desloratadine), and Xyzal (levocetirizine).

However, other experts suggest that factors like small sample size and the age of the study raise questions about the results. They suspect that Allegra, Claritin, and other newer drugs are less likely to cause weight gain than Periactin (cyproheptadine) and other first-generation drugs.

Why Do Antihistamines Cause Weight Gain?

The cause of the weight gain among antihistamine users is unclear. Some researchers theorize that antihistamines indirectly promote weight gain by increasing appetite or limiting activity through lethargy.

Other scientists contend antihistamines more directly act on cells and processes in the body. A 2019 study from Texas A&M University suggests the prolonged use of Clarinex is associated with an increased risk of high cholesterol, high blood sugar, and insulin resistance.

Animal studies have suggested that these metabolic changes cause weight gain by increasing subcutaneous fat in the abdomen and so-called "brown fat" that the body normally uses to insulate itself from cold.

Which Antihistamines May Cause Weight Gain?

Some studies find evidence that the newer (second-generation) antihistamine drugs are more likely to lead to weight gain. These drugs include:

  • Allegra
  • Zyrtec
  • Claritin (loratadine)
  • Clarinex (desloratadine)
  • Xyzal (levocetirizine)

In some cases, drugs with antihistamine properties may be used intentionally to cause weight gain. That's the case, for example, with some antihistamines used to stimulate appetite in people diagnosed with cystic fibrosis. These drugs include:

  • Periactin (cyproheptadine)
  • Remeron (mirtazapine)
  • Pizotefin

Some antihistamines are used to treat nausea and vomiting (including morning sickness) or motion sickness (as with Benadryl). They may limit weight loss due to symptom relief.

Which Antihistamines Are Least Likely to Cause Weight Gain?

Although researchers find evidence for weight gain with some of the newer antihistamines, the older drugs are still associated with weight gain. Whether or not you gain weight with certain drugs also depends on many other factors, including the use of other medications.

Generally speaking, newer drugs like Allegra and Claritin may even be less likely to cause weight gain than earlier drugs like Periactin, but research results are mixed.

How to Prevent Weight Gain From Antihistamines

Talk to your healthcare provider about whether the antihistamines you take are leading to weight gain or increased risk. They may be able to change a medication or dosage.

You also can follow general lifestyle habits meant to help people maintain a healthy weight. These include:

  • Nutritious diet, regular meals, and appropriate calorie levels
  • Exercise and fitness
  • Stress management and mindfulness practice

You also can try alternatives to antihistamines, such as vitamin C, probiotics, acupuncture, or nasal irrigation.

Summary

Antihistamines are a class of drugs that are used to treat allergy symptoms. While newer-generation antihistamines like Allegra and Zyrtec are less likely to cause drowsiness than older drugs like Benadryl, there's less clarity about an increased risk of weight gain in some users.

Some studies suggest that the prolonged use of newer-generation antihistamines can significantly increase body weight and waist circumference compared to people who don't use antihistamines. Even children who take antihistamines appear to be affected.

The cause of the weight gain is unknown. Some experts contend that antihistamines can indirectly cause weight gain by increasing appetite and lowering energy levels. Others believe that antihistamines cause metabolic changes that directly increase fat reserves in the body.

12 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Ratliff JC, Barber JA, Palmese LB, Reutenauer EL, Tek C. Association of prescription H1 antihistamine use with obesity: results from the National Health and Nutrition Examination Survey. Obesity (Silver Spring). 2010;18(12):2398-400. doi:10.1038/oby.2010.176

  2. Zuberbier T, Aberer W, Asero R, Abdul Latiff AH, Baker D, Ballmer-Weber B, et alThe EAACI/GA²LEN/EDF/WAO guideline for the definition, classification, diagnosis and management of urticariaAllergy. 2018 Jul;73(7):1393-1414. doi: 10.1111/all.13397.

  3. Wise SK, Damask C, Roland LT, Ebert C, Levy JM, Lin S, Luong A, et al. International consensus statement on allergy and rhinology: Allergic rhinitis - 2023. Int Forum Allergy Rhinol. 2023 Apr;13(4):293-859. doi: 10.1002/alr.23090.

  4. Saad M, Syed S, Ilyas M, Gashev AA. Antihistamines Increase Body Mass Index Percentiles and Z-Scores in Hispanic Children. Children (Basel). 2020 Dec 17;7(12):305. doi: 10.3390/children7120305. 

  5. MedlinePlus. Fexofenadine.

  6. Food and Drug Administration. Zyrtec (cetirizine hydrochloride) tablets and syrup for oral use.

  7. UW Medicine, University of Washington. Is Antihistamine Use Making Me Gain Weight?

  8. Gasheva OY, Nizamutdinova T, Hargrove L, et al. Prolonged intake of desloratadine: mesenteric lymphatic vessel dysfunction and development of obesity/metabolic syndrome. Am J Physiol Gastrointest Liver Physiol. 2019 Jan 1; 316(1):G217–27. doi:10.1152/ajpgi.00321.2018

  9. McTavish D, Thornton J. Appetite stimulants for people with cystic fibrosis. Cochrane Database Syst Rev. 2022 Sep 23;9(9):CD008190. doi:10.1002/14651858.CD008190.pub3.

  10. Khalili A, Sadeghi B. Chronic Urticaria in Pregnancy: Physiologic and Hormonal Background for an Immune Skin DiseaseJournal of South Asian Federation of Obstetrics and Gynaecology, (2024): 10.5005/jp-journals-10006-2381

  11. Seidman MD, Gurgel RK, Lin SY, et al. Clinical practice guideline: allergic rhinitisOtolaryngol Head Neck Surg. 2015;152(1_suppl):S1-S43. doi:10.1177/0194599814561600

  12. Luo C, Peng S, Li M, Ao X, Liu Z. The efficacy and safety of probiotics for allergic rhinitis: a systematic review and meta-analysisFront Immunol. 2022;13:848279. doi:10.3389/fimmu.2022.848279

By Daniel More, MD
Daniel More, MD, is a board-certified allergist and clinical immunologist. He is an assistant clinical professor at the University of California, San Francisco School of Medicine and formerly practiced at Central Coast Allergy and Asthma in Salinas, California.