What is Onion?

Onion is the bulb of Allium cepa, a food crop grown all over the world. In German it is called Küchenzwiebel, the kitchen onion. People eat it daily, and some also use it as a home remedy.

European folk medicine reached for onion for coughs, colds, earache, bruises and insect stings. Home cooks made syrups, poultices and raw juice. These uses are old, but long use is not proof that something works or that it is safe.

Here is the honest verdict. For most folk uses, good human evidence is missing. The best studied product is onion extract gel for scars, and even there the results are mixed.

As a food, onion is safe for most people. The main problems are heartburn, gas and bad breath. True onion allergy is rare but real.

How does Onion work?

Most of what is known about how onion acts comes from lab work, not from people.

In test tubes, the sulfur compounds in onion slow the growth of some bacteria and fungi. Whether that happens on your skin or in your gut is not established.

In cell and animal studies, quercetin from onion appears to damp down inflammation signals. It also appears to act as an antioxidant. These findings do not prove a health effect in people.

Lab studies also suggest onion compounds may be antiplatelet (they may make blood platelets less sticky, so blood clots less easily). Whether a normal food portion does this in people is not clear.

Onion extract scar gels are thought to soften scar tissue and calm redness. Human trials show small or no benefit, depending on the study and the scar.

Raw onion also contains an enzyme that makes your eyes water when you cut it. That part is simple chemistry, not medicine.

What forms does Onion come in?

Onion is used in these forms:

- Raw or cooked as food. This is by far the most common use.

- Home-made onion syrup, usually onion with sugar or honey.

- Onion juice, taken by spoon or rubbed on the scalp.

- Onion poultice, a warm cloth wrap used on the chest or skin.

- Onion extract gel or cream for scars, sold in pharmacies.

- Dried onion powder in capsules.

- Quercetin capsules. These are not onion, but they hold one of its compounds.

Food use and scar gel are the two forms with the most human data behind them.

Where it comes from:

Onion is one of the oldest cultivated food plants. It was grown in ancient Egypt and eaten across the Roman world.

In European folk practice it held a steady place in the home. Cut onion was placed in sickrooms in the belief that it drew out illness. Warm onion bags were held against a sore ear, and onion syrup was given for coughs.

Onion poultices were also laid on the chest for colds and on bruises and stings. Herbals from the medieval and early modern period list it for many complaints.

These records tell us what people did and believed. They were written long before anyone tested whether the treatments worked.

What does Onion help with?

• Scar appearance after surgery or injury: This is the best studied use. Onion extract gels are sold in many countries for new scars. Trials in people give mixed results. Some show a small gain in scar colour and softness. Others show no clear benefit over a plain moisturiser. If there is an effect, it is modest and slow. Scar gel does not remove a scar.
• Cough and common cold: European folk practice used onion syrup for coughs in children and adults. There are no good human trials to support it. Any relief may come from the sugar or honey, which do soothe a sore throat. Honey must never be given to a baby under one year. That risk comes from the honey, not the onion.
• Earache: A warm onion bag held against the ear was a common folk step for earache. There is no reliable human evidence that it helps. Warmth alone may feel soothing. Never put onion juice or onion pieces into the ear canal. Ear pain with fever or discharge needs a doctor, not a kitchen remedy.
• Insect bites and stings: Rubbing a cut onion on a sting was a standard folk move. Human evidence is essentially absent. Cool, wet contact may dull the itch for a short time. Onion juice can irritate broken skin. A cold pack is a simpler and safer choice.
• High blood pressure: Small trials have tested onion extract rich in quercetin. Pooled results suggest a small drop in blood pressure in some groups. The studies are few, short and not all agree. This is not a reason to skip blood pressure medicine. Eating onion as part of a vegetable-rich diet is sensible either way.
• Blood sugar and cholesterol: Lab and animal work hints that onion compounds affect blood sugar and blood fats. Human trials are small and mixed. No firm effect has been shown. Onion is a useful food in a healthy diet. It is not a treatment for diabetes or high cholesterol.
• Patchy hair loss: One small trial from 2002 applied raw onion juice to the scalp in alopecia areata, a condition that causes round bald patches. More people regrew hair than in the control group. The study was small and has not been firmly repeated. Onion juice smells strongly and can irritate the scalp. Treat this as an unproven idea, not an answer.

Which foods are rich in Onion?

1. Yellow and brown cooking onions
2. Red onions
3. Shallots
4. Spring onions
5. Leeks and garlic, close relatives
6. Onion soups, stocks and stews
7. Pickled onions

Practical Tips:

Onion is itself a food, so these are simply the forms you meet in the kitchen.

Red onion and the outer rings hold the most quercetin. Peeling away extra layers throws much of it out.

Boiling moves some quercetin into the water, so soups keep it. Apples, capers and tea are other common sources of the same compound.

What are the side effects of Onion?

• Bad breath and body odour
• Burping and heartburn
• Gas, bloating and loose stools
• Watering, stinging eyes when cutting it
• Skin rash or burn from onion poultices left on too long
• Contact rash on the hands in cooks and food workers
• Allergic reaction, which is rare but can be serious
• Worse symptoms in irritable bowel syndrome, a gut disorder with pain and bloating

Good to know:

Most side effects are mild and come from eating a lot of raw onion. Breath odour, gas and heartburn are by far the most common.

Skin problems come mainly from poultices and raw juice held against the skin. Onion juice can sting and, left on for hours, can blister.

True onion allergy is uncommon. When it happens it can be serious and needs urgent care.

How much Onion should you take?

• Recommended daily: Onion is a food, so there is no set dose. Diet studies often look at a quarter to one medium onion a day. Scar gel labels usually say to apply a thin layer to a closed scar a few times a day. Follow the label on the product you buy. Onion powder and quercetin capsules carry their own label amounts, often a few hundred milligrams a day. These figures are what labels state. They are not advice for you, and they are not tested doses for any disease.
• Best time of day: Timing does not matter for onion as a food. Eat it when you like it. Some people find raw onion late at night brings on heartburn. If that happens to you, shift it earlier in the day. Scar gel is usually applied morning and evening on clean, dry skin.
• With or without food: Onion is food, so this question mostly does not apply. Cooked onion in a meal is gentler on the stomach than raw onion alone. If raw onion gives you heartburn or gas, cook it. Onion powder or quercetin capsules are usually taken with a meal to reduce stomach upset.
• Time to effect: For food use, there is no effect to wait for beyond flavour and nutrition. Scar gels, if they help at all, work over months rather than days. Trials ran for eight weeks or longer. For coughs, earache or stings, any relief is immediate and short. Many people notice nothing at all, and that is a normal outcome.
• How long to take it: As food, onion can be part of your diet for life. Scar gel labels often suggest weeks to several months, since scars change slowly. Supplements have no agreed length of use. If a symptom is not getting better after about two weeks of any home use, stop and get it checked.