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Quick answer
Garlic is one of the better-studied heart-health supplements. Pooled results from randomised trials show that garlic supplements lower blood pressure by an average of around 8 to 9 mmHg systolic and 6 mmHg diastolic in people with high blood pressure, similar to a single standard blood pressure medicine at a low dose. Effects on cholesterol are small and inconsistent.
Garlic can add to the effect of blood thinners and raise bleeding risk, and it can reduce the effectiveness of some medicines. Garlic in food is safe and healthy; supplements need more care.
How garlic might work
When garlic is crushed or chopped, a compound called alliin is converted into allicin, which breaks down into a range of sulphur compounds. These may:
- Help blood vessels relax, partly by increasing nitric oxide and hydrogen sulphide production
- Slightly reduce cholesterol production in the liver
- Make blood platelets less sticky, which is why garlic affects bleeding
Aged garlic extract, made by soaking garlic for many months, contains a stable compound called S-allylcysteine, which is used in many trials.
What the research says
Blood pressure
A 2016 meta-analysis in the Journal of Nutrition combined 20 trials and found that garlic supplements reduced systolic blood pressure by about 8.6 mmHg and diastolic by about 6.1 mmHg in people with hypertension. There was little effect in people with normal blood pressure. Aged garlic extract was the most commonly studied product, and trials usually lasted 8 to 12 weeks.
A reduction of this size, if sustained, is meaningful: lowering systolic blood pressure by around 10 mmHg is linked to a substantially lower risk of heart attack and stroke. However, garlic trials were smaller and shorter than drug trials, and they did not measure heart attacks or strokes directly.
Cholesterol
Reviews suggest garlic may lower total and LDL cholesterol slightly, by around 5 to 10% in some analyses, with more effect in people whose levels are high and in longer trials. Results vary a lot between products and studies.
Other claims
Garlic has been studied for preventing colds and certain cancers, with weak or inconsistent results.
How people use it
In food: Garlic is part of a healthy Mediterranean or Middle Eastern style diet. Crushing or chopping garlic and leaving it for about 10 minutes before cooking allows more allicin to form. Food amounts are safe for nearly everyone.
As a supplement: trial doses typically included:
- Aged garlic extract: around 600 to 2,400 mg a day
- Garlic powder tablets: around 600 to 900 mg a day
If you want to try a supplement for blood pressure:
- Talk to your doctor first, especially if you already take blood pressure medicine, as the effects can add up.
- Check your blood pressure at home or at a pharmacy before starting and after 8 to 12 weeks.
- Keep taking any prescribed medicines. Garlic may be an addition, not a replacement.
- Stop garlic supplements 7 to 10 days before surgery or dental extractions, and tell your surgeon or dentist.
Possible side effects and interactions
Common side effects:
- Bad breath and body odour, which are less noticeable with aged garlic extract
- Heartburn, wind and stomach upset
- Skin irritation or burns if raw garlic is applied to skin
Interactions:
- Blood thinners and antiplatelet medicines (such as warfarin, apixaban, rivaroxaban, clopidogrel, aspirin): increased bleeding risk
- Saquinavir and some other HIV medicines: garlic can lower their blood levels and make them less effective
- Blood pressure medicines: added effect, possibly causing dizziness
- Some medicines processed by the liver, based on limited evidence
Who should avoid garlic supplements
- People taking blood thinners or antiplatelet medicines, unless their doctor agrees
- People with bleeding disorders
- People with surgery or dental procedures in the next one to two weeks
- People taking HIV medicines
- Pregnant or breastfeeding women (food amounts are fine)
- People with stomach ulcers or bad reflux may find garlic irritating
When to see a doctor
High blood pressure usually has no symptoms, so regular checks are important. See a doctor if:
- Your home readings are regularly 135/85 mmHg or higher, or a clinic reading is 140/90 or higher
- You have a reading of 180/120 or higher, especially with headache, chest pain, shortness of breath, vision changes or confusion. Seek urgent care.
- You have unusual bruising, nosebleeds or bleeding gums while taking garlic supplements
- You have chest pain, breathlessness on exertion, or palpitations
Frequently asked questions
Is raw garlic better than supplements?
Raw garlic forms allicin when crushed, but the amount varies and many people can't eat enough comfortably. Most trials used standardised supplements. Both can be part of a healthy diet.
How long does garlic take to lower blood pressure?
In trials, effects appeared over 8 to 12 weeks of daily use.
Can garlic replace my blood pressure medicine?
No. Never stop prescribed medicine without talking to your doctor.
Does garlic help bleeding gums?
No, and it may make bleeding slightly worse in people on blood thinners. Bleeding gums are usually a sign of gum disease and need a dental check.
References
- National Center for Complementary and Integrative Health (NCCIH). Garlic. https://www.nccih.nih.gov/health/garlic
- Ried K. Garlic lowers blood pressure in hypertensive individuals, regulates serum cholesterol, and stimulates immunity: an updated meta-analysis and review. Journal of Nutrition, 2016.
- Ried K, Toben C, Fakler P. Effect of garlic on serum lipids: an updated meta-analysis. Nutrition Reviews, 2013.
- Ettehad D, et al. Blood pressure lowering for prevention of cardiovascular disease and death: a systematic review and meta-analysis. The Lancet, 2016.