Onion (Allium cepa) is a culinary and medicinal herb. It belongs to the Allium genus along with garlic, shallots, leeks, scallions, and chives.

It is believed to be one of the earliest cultivated vegetables in the world. In fact, it’s been used by the Romans, the Greeks, and many other cultures for thousands of years as food and as a remedy for numerous ailments (1).

What’s more, onion’s use as an herbal remedy led modern research to discover its many medicinal benefits. For example, it may have anti-inflammatory, antibacterial, antithrombotic, anticancer, antioxidant, and heart- and liver-protective properties (1).

Still, despite this plethora of potential health benefits, onions may cause unwanted side effects in some people.

This article reviews some of onion’s most common potential downsides.

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Irritable bowel syndrome (IBS) is a condition that affects 10–15% of the U.S. population. It’s characterized by symptoms like bloating, abdominal pain, cramping, gas, generalized weakness, and abnormal bowel movements varying from constipation to diarrhea (2, 3, 4).

IBS treatment involves a multimodal approach focused on managing symptoms to improve quality of life. Dietary changes are a key factor (3, 5).

While foods are not considered a cause of IBS, they may trigger its symptoms. Therefore, a low FODMAP diet has become a popular evidence-based treatment (2, 5, 6).

FODMAP” stands for “fermentable oligo-, di-, monosaccharides and polyols.” These are indigestible and slowly absorbed short-chain carbohydrates that are found in certain foods and cause discomfort in people with IBS (5, 6).

A low FODMAP diet aims to avoid foods high in FODMAPs and replace them with foods low in FODMAPs to minimize intestinal swelling. Onions, garlic, shallots, legumes, coconut, and milk- and wheat-based products are common high FODMAP foods (5, 6).

Since onions may trigger or worsen IBS symptoms, some people may have to temporarily or permanently avoid them.

Summary

Onions are a high FODMAP food, meaning they can worsen IBS symptoms in some people.

Food allergies occur when your immune system has an adverse reaction to proteins from certain foods. This immune response can affect organs and systems such as your skin, gut, heart, and respiratory and nervous systems (7, 8).

The most common food allergens for kids and adults are cow’s milk, eggs, peanuts, fish, shellfish, tree nuts, soybeans, and wheat. Further, fruits and vegetables are prevalent allergens in adults (7).

A few studies have identified allergic reactions to onions. Onions belong to the Liliaceae family of flowering plants, along with garlic, asparagus, and leeks (8).

Research suggests that allergenic cross-reactivity may occur among different Liliaceae plants as well as grass pollens. This means that if you’re allergic to one of them, you may also experience allergic reactions to others (8, 9).

Onions contain compounds called diallyl disulfide and lipid transfer protein, which can cause allergy symptoms like asthma, runny nose, nasal congestion, red eyes, itchy eyes and nose, and contact dermatitis, characterized by a red, itchy rash (9, 10).

In rare cases, contact dermatitis can occur when people simply handle onions without eating them (9).

Summary

While it is rare, some people may experience allergic reactions when handling or ingesting onions.

When chopping onions, you’ve likely experienced a stinging sensation in your eyes that causes them to tear up.

Members of the Allium genus produce a sulfur metabolite called lachrymatory factor (LF), which works as a chemical defense mechanism against animals and microbes (11, 12).

LF is a volatile compound, which means it evaporates easily. It’s released as a result of a series of enzymatic reactions that occur when onion tissue is disrupted — such as when you chop it (11, 12, 13).

Because of its volatility, LF can reach your eyes and cause the irritation responsible for onion’s tear-inducing effect (11, 12, 13).

To prevent LF from reaching your eyes, you can try chilling the onion before handling it, chopping it under running water, wearing sealed goggles, and lighting a match (13).

Summary

Chopping onions leads to the release of LF, a gas that irritates your eyes and causes tear production.

Heartburn can be a symptom of gastroesophageal reflux disease (GERD) — a condition in which the contents of your stomach travel back up into your esophagus, the tube connecting your stomach and throat. This can cause a burning sensation in your chest (14).

It happens when the muscle at the end of your esophagus, called the lower esophageal sphincter (LES), relaxes. This can allow food and gastric acid to pass into your esophagus (15).

Research from 2000 suggests that some foods, including onions, may trigger heartburn symptoms by promoting LES relaxation (16).

A study from 1990 observed that a meal containing onions significantly increased heartburn episodes in people with frequent heartburn, both when compared with the same meal without onions and with a control group of people with no heartburn (17).

Therefore, you may want to skip onions if you have heartburn.

Summary

Onions may promote LES relaxation and trigger heartburn symptoms, especially in people with GERD.

Research has linked onion intake to a few other potential side effects.

Bad breath

If you’ve ever enjoyed a dish with onion, this smelly side effect likely won’t surprise you.

Consuming certain foods, such as onions and garlic, may cause temporary halitosis, more commonly known as bad breath (18).

If you’d like to enjoy a dish with onion but worry about bad breath, you may be able to combat the smell by brushing your teeth, flossing, chewing gum, or rinsing with mouthwash after eating (18).

Drug interaction

Onions have a potential anticoagulant effect, meaning that they may help prevent the formation of blood clots. While this might sound like a benefit, it can be an issue, especially if you’re taking certain medications.

For example, both older and more recent studies show that onions may interfere with anticoagulant drugs like warfarin and moderately increase your risk of bleeding (19, 20).

Still, these studies fail to mention the quantities needed to pose a health risk, so more research is needed.

Summary

Onions may increase your risk of having bad breath. They may also interfere with anticoagulant drugs like warfarin.

Onions are a versatile vegetable used in numerous dishes across many cuisines. They have also been used throughout history as a home remedy due to their multiple health benefits.

Unfortunately, onions also have several downsides. These range from mild, such as bad breath and eye irritation, to severe, like allergic reactions and drug interactions.

Still, most people can enjoy onions in their cooking with minimal or no issues. So, if you’ve never experienced any downsides from eating this popular vegetable, there’s no reason to stop now.