Garlic has been a staple of traditional medicine for thousands of years. Long before pharmaceutical companies existed, people across cultures used garlic to treat infections, digestive complaints, and circulatory problems. The ancient Egyptians fed it to pyramid builders, Greek athletes ate it before competitions, and medieval Europeans believed it could ward off plague.

The main active compound in garlic is allicin, a sulphur-containing substance that forms when raw garlic is crushed, chopped, or chewed. This compound is responsible for garlic’s pungent smell and many of its biological effects. Allicin is unstable and quickly converts to other sulphur compounds, which is why the form of garlic (raw, cooked, aged, or supplemental) matters for its health effects.

Modern research has investigated garlic’s traditional uses with varying results. Some claims hold up well under scientific scrutiny; others remain unproven or show only modest effects. Here’s what the current evidence actually shows.

1. Cardiovascular Protection

Cardiovascular disease remains the leading cause of death globally, responsible for roughly 17.9 million deaths annually according to the World Health Organization 1. Any dietary intervention that might reduce this burden attracts considerable interest.

Garlic appears to offer several cardiovascular benefits working through different mechanisms:

Reduced arterial calcification: Aged garlic extract has been shown to slow the progression of coronary artery calcium, a marker of atherosclerosis. One study found participants taking aged garlic extract showed significantly less progression of calcium deposits compared to placebo over 12 months 2.

Anti-platelet effects: A compound in garlic called ajoene interferes with platelet aggregation, potentially reducing the risk of blood clots. This effect is similar to (though weaker than) aspirin, which is widely used for cardiovascular prevention 3.

Cholesterol reduction: Multiple studies suggest garlic can lower total cholesterol and LDL cholesterol by approximately 10-15%. A meta-analysis of 39 trials found garlic preparations reduced total cholesterol in those with elevated levels, though the effect was modest 4.

Improved endothelial function: Garlic appears to increase nitric oxide production in blood vessel walls, promoting vasodilation and improved blood flow 5.

The catch here is that these effects, while real, are relatively modest. Garlic won’t replace proper medical treatment for established heart disease, but it may contribute to overall cardiovascular health as part of a broader dietary approach.

2. Blood Pressure Reduction

Hypertension affects over a billion people worldwide, and roughly half of those affected don’t know they have it because symptoms are often absent until serious damage occurs. The NHS estimates that high blood pressure causes about 75,000 deaths annually in the UK alone 6.

A meta-analysis of 17 randomised controlled trials found that garlic supplementation produced a statistically significant reduction in blood pressure, with average drops of 3.75 mmHg systolic and 3.39 mmHg diastolic 7. The effect was more pronounced in people with existing hypertension than in those with normal blood pressure.

To put this in perspective, a reduction of 3-4 mmHg in systolic blood pressure is meaningful at a population level but modest for an individual. It’s roughly equivalent to what you might achieve through moderate salt restriction. For someone with significantly elevated blood pressure, garlic supplements alone won’t bring readings into the normal range, but they could be a useful adjunct to other lifestyle changes and, where necessary, medication.

The mechanism likely involves garlic’s effects on nitric oxide production and angiotensin-converting enzyme (ACE) inhibition. Different garlic preparations (powder, extract, aged extract) have shown similar effects, suggesting the benefit isn’t limited to one particular form.

3. Immune Function and Common Colds

The idea that garlic fights infections isn’t just folklore. Garlic does have genuine antimicrobial properties in laboratory settings, active against various bacteria, viruses, and fungi. Whether this translates to meaningful clinical benefits is more complicated.

One frequently cited study published in Advances in Therapy found that participants taking a garlic supplement containing allicin experienced 63% fewer colds over a 12-week period compared to placebo. Those who did catch colds recovered faster, with an average duration of 1.5 days compared to 5 days in the placebo group 8.

These results are impressive, perhaps too impressive. The study was relatively small (146 participants) and has been difficult to replicate. A Cochrane review noted that while garlic may reduce cold frequency, the evidence base is limited and more high-quality trials are needed 9.

My take: garlic probably has some immune-supportive effects, but I wouldn’t bet the farm on it preventing your next cold. If you enjoy garlic in your cooking, there’s no harm in continuing to eat it during cold season. Whether it’s worth taking concentrated supplements specifically for immune support is less certain.

4. Cognitive Function and Dementia Prevention

Alzheimer’s disease and other forms of dementia represent one of the most feared aspects of ageing. The brain changes associated with Alzheimer’s, including accumulation of amyloid plaques and tau tangles, begin decades before symptoms appear.

Garlic, particularly aged garlic extract, contains compounds with antioxidant and anti-inflammatory properties that, in theory, could protect brain cells from damage. Animal studies have shown that garlic compounds can prevent amyloid plaque formation and improve cognitive function in mouse models of Alzheimer’s disease 10.

Human evidence is more limited. Some observational studies suggest people who regularly consume garlic have lower rates of cognitive decline, but this could reflect other healthy lifestyle factors rather than garlic specifically. Clinical trials examining garlic’s effects on cognitive function in humans are few and generally small.

If you’re interested in dietary approaches to brain health, garlic might be one component of a broader strategy. The Mediterranean diet, which typically includes generous amounts of garlic along with olive oil, fish, and vegetables, has shown more consistent associations with preserved cognitive function.

5. Bacterial Vaginosis

Bacterial vaginosis is a common condition affecting nearly 30% of women at some point, caused by an imbalance in vaginal flora. The typical treatment is antibiotics, but recurrence rates are frustratingly high, and some women seek alternative approaches.

One controlled study compared garlic extract to the standard antibiotic metronidazole in 120 women with bacterial vaginosis. After seven days, the garlic group showed a 70% reduction in diagnostic criteria compared to 48% in the antibiotic group, with no significant difference in overall treatment success between groups. The antibiotic group experienced more gastrointestinal side effects 11.

This is genuinely interesting, but it’s a single study with a relatively short follow-up period. Bacterial vaginosis frequently recurs regardless of initial treatment success. More research would be needed before recommending garlic as a primary treatment. For women experiencing recurrent bacterial vaginosis who haven’t responded well to standard treatments, it might be worth discussing with a healthcare provider as a potential option.

6. Oral Health

Dental plaque, the bacterial film that accumulates on teeth, causes bad breath, cavities, gum disease, and eventually tooth loss if left untreated. The antibacterial compounds in garlic have been studied for their effects on oral pathogens.

Laboratory studies confirm that garlic extracts can inhibit the growth of bacteria commonly found in dental plaque, including Streptococcus mutans, a primary culprit in tooth decay 12. Whether eating garlic or using garlic-containing mouthwashes provides meaningful clinical benefit is less clear.

The irony, of course, is that garlic causes breath that many people find offensive, somewhat undermining its potential oral health benefits. Raw garlic breath can persist for hours after consumption as the sulphur compounds are absorbed into the bloodstream and released through the lungs.

Worth noting: some garlic supplements, particularly aged garlic extract, produce less odour than raw garlic while potentially retaining beneficial compounds.

7. Heavy Metal Detoxification

Lead exposure remains a public health concern in many parts of the world. Sources include old paint, contaminated water pipes, certain occupational exposures, and environmental pollution. Chronic lead exposure causes kidney damage, cognitive impairment, hypertension, and reproductive problems.

A study among battery factory workers, a population with significant lead exposure, found that garlic supplementation reduced blood lead levels by 19% and improved symptoms of lead toxicity including headaches, irritability, and elevated blood pressure. Remarkably, garlic performed comparably to the pharmaceutical chelating agent D-penicillamine 13.

This is fascinating research, though the practical applications are limited. Most people in developed countries don’t have clinically significant lead exposure. For those who do, garlic might serve as a supportive measure, but serious lead poisoning requires proper medical management with established chelation therapy.

Side Effects and Safety Considerations

Garlic is generally considered safe when consumed in normal food amounts. The US FDA classifies it as “generally recognised as safe” (GRAS). However, concentrated supplements can cause problems for some people.

Common side effects include gastrointestinal upset (heartburn, nausea, diarrhoea, flatulence), the infamous garlic breath and body odour that’s unavoidable with raw garlic, and skin irritation from handling it directly. Some people also report headaches or dizziness with high doses.

Important interactions and contraindications:

Garlic has blood-thinning properties, which can be problematic in certain situations. If you’re taking anticoagulant medications like warfarin, or other supplements with blood-thinning effects such as ginkgo, fish oil, nattokinase, or high-dose vitamin E, adding concentrated garlic supplements could increase bleeding risk.

Stop garlic supplements at least two weeks before scheduled surgery to reduce the risk of excessive bleeding during and after procedures.

Pregnant and breastfeeding women can safely consume garlic in food amounts. The safety of concentrated supplements during pregnancy hasn’t been adequately studied, so it’s generally advisable to stick to dietary amounts.

People with bleeding disorders or those about to undergo dental procedures should also exercise caution with concentrated garlic supplements.

Practical Recommendations

If you’re considering garlic for health benefits, the first question is whether to eat it or take supplements. Culinary use is safe for most people and likely provides some benefit, though for therapeutic effects, studies typically use concentrated extracts at doses equivalent to several cloves daily.

Raw versus cooked matters because heat destroys allicin. Raw garlic contains more active compounds than cooked, though aged garlic extract—which goes through a lengthy ageing process—contains different beneficial compounds despite having less allicin. If the characteristic smell puts you off, aged garlic supplements are marketed as “odourless” and may be easier to take regularly.

Not all garlic supplements are created equal. They vary considerably in allicin content and overall quality, so products standardised for allicin or S-allyl cysteine are more likely to deliver consistent amounts of active compounds. Cheaper products often contain minimal amounts of the compounds that actually do anything.

References

The claims in this article are supported by peer-reviewed research. Key sources include studies published in PubMed, guidance from the NHS, and WHO statistics on cardiovascular disease. Where possible, meta-analyses and systematic reviews have been prioritised over individual studies.