The short answer: garlic is one of the most concentrated sources of fructans in the everyday diet. Humans have no enzyme to break fructans down, so they travel intact to the large intestine where bacteria ferment them, producing gas, distension and cramping. Separately, garlic relaxes the valve at the top of the stomach, which is why the same meal can cause heartburn. Neither of these is an allergy, and neither means anything is wrong with you.

What in garlic actually causes the trouble?

Garlic contains two quite separate groups of compounds, and people routinely blame the wrong one.

The famous group is the sulphur compounds. Alliin converts to allicin when a clove is crushed, and allicin breaks down further into the compounds responsible for the smell, the taste and most of garlic's studied effects. These are responsible for garlic breath and for skin irritation, but they are not the main reason your stomach objects.

The group that causes the digestive symptoms is the carbohydrates. Garlic is rich in fructans, chains of fructose units also known as inulin type fructans. Humans produce no enzyme capable of splitting those chains, so unlike starch or table sugar they pass through the small intestine essentially untouched.

Two things then happen. Fructans are osmotically active, so they draw water into the bowel. And when they reach the colon, the resident bacteria ferment them enthusiastically, producing hydrogen, carbon dioxide and in some people methane. More water plus more gas equals distension, audible gurgling, cramping and wind, typically somewhere between two and eight hours after eating, which is late enough that most people never connect it to the meal.

This is a normal physiological response, not a malfunction. The difference between people who barely notice and people who are doubled over is mostly a matter of visceral sensitivity, how loudly the gut reports normal distension to the brain, which is the central feature of irritable bowel syndrome.

Garlic, FODMAPs and IBS

FODMAP is an acronym for a family of short chain carbohydrates that share this property of being poorly absorbed and readily fermented. Fructans are the F. Irritable bowel syndrome affects a substantial share of UK adults, and when people work through a structured low FODMAP diet, garlic and onion emerge repeatedly as the two foods most often responsible.

Three practical points that get lost in the online version of this advice.

Cooking does not help. Fructans are heat stable. Slow roasted garlic tastes sweet and mild because the sulphur chemistry has changed, and that mellowness fools people into thinking it has become gentler on the gut. The fructan content is essentially unchanged.

Dose matters enormously. This is not an all or nothing intolerance. Most people have a threshold, and the same person who reacts to three cloves in a curry may be entirely fine with the trace amount in a stock. It also stacks across a meal, so garlic bread alongside an onion soup and a wheat pasta dish is three fructan sources at once.

A low FODMAP diet is a diagnostic tool, not a way of life. It is designed as a short elimination of a few weeks followed by systematic reintroduction to find your personal thresholds, and it is genuinely worth doing with a dietitian. Staying on the elimination phase indefinitely narrows your diet and reduces the fibre your gut bacteria depend on, which tends to make things worse in the long run.

The infused oil trick, and the safety rule that goes with it

Here is the piece of information that changes most people's cooking. Fructans are water soluble. They are not oil soluble. The aromatic sulphur compounds that give garlic its flavour largely are oil soluble.

So if you gently heat garlic in oil and then remove and discard the solids, you get an oil that tastes of garlic and contains essentially no fructans. For a great many people who thought they had to give up garlic entirely, this is the whole answer. The same logic applies to onion, and to the green tops of spring onions and leeks, which are far lower in fructans than the white bulbs.

It does not work in reverse. Garlic simmered in a stock, a soup or a sauce leaches its fructans straight into the water, so fishing the cloves out at the end achieves nothing.

Reflux and heartburn are a different problem

If your symptom is burning behind the breastbone, an acid or bitter taste, or a cough that is worse lying down, that is reflux rather than fermentation, and the mechanism is unrelated to fructans.

The lower oesophageal sphincter is the muscular ring that keeps stomach contents where they belong. Garlic, along with onion, chocolate, peppermint, caffeine, alcohol and fatty meals, is a recognised relaxer of that sphincter. Raw garlic on an empty stomach is also directly irritating to the stomach lining for many people, and concentrated garlic supplements are considerably worse than food, which is the most common reason people quietly abandon them.

What tends to help: eat garlic as part of a meal rather than raw and alone, cook it rather than eating it raw, reduce portion size, avoid eating within three hours of lying down, and raise the head of the bed rather than piling up pillows. If you take garlic capsules, an enteric coated version taken with food is usually better tolerated.

Reflux that needs a GP rather than a diet change: symptoms most days for more than three weeks, difficulty or pain on swallowing, food sticking, vomiting, unintentional weight loss, or anaemia. And if you are reaching for antacids constantly, that is worth a conversation rather than a bigger box.

Is it an allergy or an intolerance?

Almost always an intolerance, and the distinction matters because the two are managed completely differently.

There is a third, quite separate problem worth knowing about if you cook a lot. Diallyl disulfide in raw garlic is a well recognised cause of allergic contact dermatitis on the fingertips, classically the thumb, index and middle finger of the non dominant hand, the hand that holds the clove. Chefs get it often. Raw garlic held against the skin under a plaster, a folk remedy for various things, causes genuine chemical burns, which is one of several reasons we advise against kitchen garlic remedies applied to skin or scalp.

Why brushing your teeth does not fix garlic breath

Because most of it is not coming from your mouth. The immediate smell after eating is oral and does respond to brushing. The version that is still with you the following morning is allyl methyl sulfide, a compound produced when garlic is metabolised, absorbed into the bloodstream and then released through the lungs and the skin over up to 24 hours. You are, in effect, exhaling it. No amount of toothpaste reaches the bloodstream.

What has actually been tested with some success: raw apple, raw lettuce and fresh mint, which contain enzymes and polyphenols that appear to break down the volatile sulphur compounds, and milk, where the fat and protein seem to do the work. Whole milk performs better than skimmed, and drinking it during rather than after the meal works better. Cooking garlic thoroughly also reduces the effect considerably compared with raw.

What about the big health claims?

Garlic is a genuinely good food. It is also credited with a great deal it cannot do, and the version of this article we replaced claimed garlic fights cancer, improves insulin resistance, treats arthritis, boosts athletic endurance and prevents arterial plaque. Here is the honest position on each.

For the fuller picture of what garlic does and does not do, our main guide to garlic health benefits and what the evidence actually supports is the place to start.

Who should be careful with garlic

• Anyone taking warfarin, a DOAC, aspirin or clopidogrel, because concentrated garlic supplements can add to bleeding risk.
• Anyone due surgery or a dental extraction, who should usually stop garlic supplements seven to ten days beforehand and declare them.
• People taking the HIV medicine saquinavir, whose levels garlic supplements substantially reduce.
• People with reflux, gastritis or a stomach ulcer, particularly with raw garlic or high dose capsules.
• People following a low FODMAP diet under dietitian supervision.
• People who have had a genuine allergic reaction, who should avoid it entirely and be assessed.

Frequently asked questions

Why does garlic make me bloated?

Garlic is high in fructans, a carbohydrate humans cannot digest. It passes intact to the large intestine, draws water in, and is fermented by gut bacteria into gas. That combination causes distension, cramping and wind, usually two to eight hours after the meal.

Does cooking garlic make it easier to digest?

Not for bloating. Fructans are heat stable, so roasting or slow cooking changes the flavour but not the fructan content. Cooking does reduce the sulphur compounds, so it helps with breath and with stomach irritation, just not with wind.

Can I eat garlic on a low FODMAP diet?

Garlic itself is restricted during the elimination phase, but garlic infused oil is allowed, because fructans are water soluble and do not transfer into oil. Green spring onion tops and the green parts of leeks are also lower in fructans than the bulbs.

Is garlic infused oil safe to make at home?

Only if you refrigerate it. Garlic carries Clostridium botulinum spores from the soil, and oil provides the oxygen free, low acid conditions those spores need to produce toxin. Keep homemade garlic oil in the fridge, use it within about four days or freeze it, and never leave it out at room temperature.

Why does garlic give me heartburn?

Garlic relaxes the lower oesophageal sphincter, the muscular valve that keeps stomach contents down, and raw garlic also irritates the stomach lining directly. This is a different mechanism from bloating, so avoiding fructans will not fix it.

Am I allergic to garlic?

Probably not. True garlic allergy is rare and causes rapid symptoms such as hives, swelling or wheeze within minutes to an hour. Symptoms hours later, dose dependent, and confined to the gut are intolerance. Seek emergency help for any throat or breathing symptoms.

How do I get rid of garlic breath?

Brushing only addresses the mouth. The lingering version is allyl methyl sulfide released through your lungs for up to 24 hours. Raw apple, raw lettuce, fresh mint and whole milk consumed during the meal have all shown some effect, and cooking garlic thoroughly reduces it substantially.

Do garlic supplements upset your stomach more than food?

Usually yes. Concentrated capsules deliver far more at once than a meal does, and heartburn, nausea and wind are the most commonly reported side effects. Taking an enteric coated form with food rather than on an empty stomach helps.

This article is general information about food and digestion and is not medical advice. Please see a GP before restricting your diet, and urgently if you have unexplained weight loss, blood in your stool, difficulty swallowing, persistent vomiting or a lasting change in bowel habit.

Watermans makes vegan, UK made hair care and has sold over 5 million bottles since 2012. We do not sell supplements, digestive products or anything else relevant to this page, so there is nothing for sale here, only the infused oil trick and the fridge rule that goes with it.