Are there any side effects of eating onions?

December 7, 2025 •

4 min read

While onions are a kitchen staple and a source of beneficial nutrients like antioxidants and vitamin C, they are not without potential downsides. For many, consuming this allium vegetable is completely harmless, but for sensitive individuals, certain compounds in onions can trigger a variety of side effects, ranging from mild digestive upset to more serious allergic reactions.

Quick Summary

Potential risks of consuming onions include gastrointestinal issues like bloating and gas, bad breath, and possible allergic reactions due to specific plant compounds. Some people with conditions such as IBS or GERD may experience aggravated symptoms, particularly from raw onions.

Key Points

Digestive Discomfort: The fructans in onions are a type of FODMAP that can cause bloating, gas, and stomach cramps, especially for those with IBS.
Allergic Reactions and Intolerances: True onion allergies are rare, but intolerances are more common and can cause symptoms like digestive upset, skin rashes, or swelling, with cross-reactivity to other alliums being possible.
Heartburn and Acid Reflux: Raw onions, in particular, can relax the lower esophageal sphincter and trigger or worsen symptoms of acid reflux and heartburn.
Bad Breath and Body Odor: The high sulfur content in onions leads to volatile compounds being absorbed into the bloodstream and released through breath and sweat, causing a lingering odor.
Medication Interactions: Excessive onion consumption can have blood-thinning effects and may interfere with anticoagulant drugs or medications for blood sugar control, requiring caution for those on such treatments.
Raw vs. Cooked: Cooking onions can reduce their fructan content and potency of sulfur compounds, often making them easier to digest and less likely to cause irritation compared to raw ones.
Moderation is Key: For most people, consuming onions in moderation is safe; however, those with known sensitivities or specific health conditions should pay close attention to portion sizes and preparation methods.

Common Digestive Issues from Eating Onions

One of the most frequently reported side effects of eating onions, especially when raw, is digestive distress. This is largely due to their high content of fructans, a type of fermentable carbohydrate known as a FODMAP (Fermentable Oligo-, Di-, Mono-saccharides And Polyols). While fructans act as beneficial prebiotics for many, feeding healthy gut bacteria, they can cause problems for those with sensitivities.

Bloating and Gas: For individuals who have difficulty absorbing fructans in their small intestine, these carbohydrates pass into the large intestine where they are fermented by gut bacteria, producing gas that leads to bloating and flatulence.
Stomach Cramps and Diarrhea: The fermentation process and fluid changes in the intestines can also cause abdominal pain, cramping, and, in some cases, diarrhea.
Heartburn and Acid Reflux: Onions can cause or worsen acid reflux symptoms, particularly in those with gastroesophageal reflux disease (GERD). They can relax the lower esophageal sphincter, allowing stomach acid to flow back into the esophagus and causing a burning sensation. Raw onions are more likely to cause this than cooked ones.

Allergic Reactions and Intolerances

While a true, IgE-mediated onion allergy is rare, intolerances are more common and cause unpleasant physical reactions. The distinction lies in the immune system's response: an allergy involves the immune system, while an intolerance involves the digestive system.

Symptoms of Allergic Reaction

For those with a true onion allergy, symptoms can be more severe and include:

Hives or rashes
Swelling of the lips, face, or tongue
Itching or tingling in the mouth
Nasal congestion or difficulty breathing
In very rare cases, anaphylaxis

Cross-Reactivity

Because onions belong to the allium genus, along with garlic, chives, shallots, and leeks, individuals sensitive to one may also react to others due to cross-reactivity. This can make dietary management more complex.

Comparison of Raw vs. Cooked Onions

The method of preparation can significantly affect the side effects experienced, especially concerning digestive comfort.


Feature	Raw Onions	Cooked Onions
Digestive Impact	Higher fructan content, more likely to cause gas and bloating.	Cooking reduces fructan levels, making them easier to digest for sensitive individuals.
Acidity	More acidic and more likely to trigger heartburn or acid reflux.	Less likely to cause acid reflux symptoms.
Sulfur Compounds	More potent, leading to stronger bad breath and potential body odor.	Cooking breaks down these compounds, diminishing the lingering odors.
Blood-Thinning	Potentially higher effect on blood clotting due to quercetin content.	Effects are significantly reduced after cooking.
Allergens	Some allergens may be more potent in raw form; some individuals only react to raw onions.	Cooking can denature proteins, potentially reducing allergic reactions for some.

Interaction with Medications

For certain individuals, eating onions can interfere with prescribed medications. Onions contain natural compounds that can have a blood-thinning effect. For those on anticoagulant drugs like warfarin, excessive onion consumption, particularly green onions, may pose a risk and warrants medical consultation. The quercetin in onions can potentially decrease blood clotting, increasing the risk of bruising or bleeding. Additionally, for diabetics taking blood sugar medication, large amounts of onion may lower blood sugar levels, requiring careful monitoring.

Bad Breath and Body Odor

The pungent aroma of onions is a well-known side effect that affects many. The sulfur compounds that give onions their distinct flavor are absorbed into the bloodstream after consumption. These compounds are then released through the lungs, causing bad breath, and also excreted through the pores in sweat, leading to a noticeable body odor. This effect can be quite persistent and is often more pronounced after eating raw onions. Managing onion breath can involve dental hygiene, chewing gum, or drinking green tea, but the effect may linger for hours.

Conclusion: Moderation is Key

In summary, while onions are a healthy and flavorful addition to most diets, they are not without potential side effects for everyone. Digestive discomfort, including gas, bloating, and heartburn, is common, especially with raw onions, due to their fructan content. Allergic reactions, though rare, can occur and may be severe, while intolerances are more widespread. Interactions with certain medications, such as blood thinners, and issues with bad breath and body odor are also valid concerns. For those with sensitivities or pre-existing health conditions, managing portion sizes, opting for cooked instead of raw onions, and consulting a healthcare provider can help mitigate any adverse effects. By understanding these risks, individuals can continue to enjoy onions safely as part of a balanced diet.

Sources

Healthline: Onions 101: Nutrition Facts and Health Effects
WebMD: Onions: Health Benefits and Nutrition
Zee News: 7 Health Risks And Side Effects Of Eating Raw Onions
Health.com: What Happens to Your Body When You Eat Onions Regularly
Medical News Today: Onion Allergy: Symptoms, Causes, Treatment

Lists

How Cooking Onions Affects Digestion: Cooking can significantly reduce the fructan content in onions, making them easier to digest for those with sensitivities.
Ways to Reduce Eye Irritation: To minimize the tearing caused by sulfur compounds when chopping, try chilling onions, cutting them under running water, or leaving the root end intact.
Onion Alternatives for Sensitivity: For individuals with an intolerance, substituting onions with fresh herbs like parsley, the green parts of scallions, or small amounts of garlic-infused oil can provide flavor without the discomfort.
Allergen Avoidance: People with a diagnosed onion allergy must be vigilant about reading food labels, as onions can be a hidden ingredient in many processed and pre-packaged foods.

Outbound Link

For further information on FODMAPs and digestive health, a great resource is Monash University's Low FODMAP Diet Blog.

Conclusion

While onions offer a range of nutritional benefits, they are not universally tolerated without consequences. Awareness of potential digestive distress, allergic reactions, and other factors is crucial for sensitive individuals. By paying attention to how your body reacts to both raw and cooked versions, and consulting a doctor for any persistent issues, you can navigate your diet effectively and enjoy the culinary benefits of this versatile vegetable with minimal risk.

Frequently Asked Questions

Onions are high in fructans, which are a type of carbohydrate that can be difficult for some people to digest. When these fructans reach the large intestine, gut bacteria ferment them, producing gas and leading to bloating.

Yes, although a true onion allergy is rare. It is more common to have an intolerance, which causes digestive issues. A true allergy is an immune system response with symptoms like hives, swelling, or breathing issues.

Generally, yes. Cooking onions can reduce the levels of fructans and sulfur compounds, making them less likely to cause digestive discomfort, bad breath, or irritate the esophageal sphincter compared to raw onions.

Onions contain sulfur compounds. When you eat them, these compounds are absorbed into your bloodstream and released through your lungs (causing bad breath) and sweat glands (causing body odor).

Yes. In large quantities, onions can have a mild blood-thinning effect and may interact with anticoagulant medications. They may also affect blood sugar levels, which is a consideration for diabetics.

Individuals with irritable bowel syndrome (IBS), gastroesophageal reflux disease (GERD), diagnosed onion allergies, or those taking blood-thinning medications should be cautious with their onion consumption.

Yes. Since symptoms of onion intolerance can be delayed and mimic other digestive issues, it can be confused with other food sensitivities or underlying conditions. Keeping a food diary and consulting a healthcare professional is recommended for a proper diagnosis.