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Garlic and its health benefits

A healthy spice

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First published: 11.Oct.2023


Garlic is consumed as a spice and ingredient in different cuisines around the world. Traditional and complementary medicine include garlic as one of their natural remedies. Science shows that it has interesting health promoting sulfur-based chemical compounds.

This article will explore its potential benefits as a treatment for cancer, high blood pressure, diabetes, blood lipids, skin conditions and liver health, as well as its risks, side effects and recommended daily doses.

In this Article (Index)

garlic cloves on a wooden table
Garlic, food ingredient with health benefits


Garlic (Allium sativum) is a member of the lily family. It is related to the onion, chive, leek and chalot. It originated in South and Central Asia and spread around the world.

It is a perennial bulbous plant that flowers in late summer. Its underground bulb consists of 10 to 20 cloves individually seathed by a leaves.

Garlic has a long history, and has been used as a seasoning for food flavoring and in traditional medicine in Egypt, Chine, Sumer and ancient Greece and Rome.

Greek pharmacologist, botanist and physician Pedanius Dioscorides (c. 40–90 AD) is known as "the father of pharmacognosy." He wrote about garlic in his medical encyclopedia De materia medica (1).
Discorides mentions its medical properties for treating those bitten by snakes, mad dogs, shrew mice, and to clear the throat, arteries, as a treatment for lice an nits (if drank as a decoction), with salt and oil, it can cure pimples, dandruff, psoriasis, ulcers, vitiligo and when boiled with pine pitch and frankincense, as mouthwash for toothache.

Nowadays it is promoted as a nutraceutical and dietary supplement for conditions such as high cholesterol, high blood pressure, heart-related conditions, osteoporosis, diabetes and cancer.

We will look into these claims and what science has to say about them, as well as the potential nasty side effects of garlic.

Below we summarize the findings of a revision of scholarly articles published between 2000 and 2020 including clinical trials involving garlic as a treatment for a wide range of diseases (2).

Types of Garlic

Garlic is consumed fresh, in foods and in traditional medicine; fresh garlic, garlic powder and garlic oil are used as spices.

Garlic can be ingested or applied topically to the skin.

Aged garlic extract, or AGE is obtained by storing sliced garlic in a 15 to 20% ethanol (grain alcohol) in water solution for approximately 18 months. This process transforms allicin into other compound.

Garlic oil is obtained by steam-distillation and contains allylic mono to hexa sulfides.

The Bioactive ingredients in Garlic

Garlic has a potent aroma and strong flavor which come from its organosulfur compounds, but it also contains alkaloids, phenolic diterpenes, polyphenols, flavonoids, tannins and vitamins.

These compounds have a low bioavailability meaning that the body has difficulty in absorbing them and rapidly eliminates them through urine. Some sulfur compounds like its allyl thiosulfates are metabolized into other substances like allyl methly sulfide, that gives the typical breath aroma characteristic of those who consume garlic.

Compounds found in Garlic

  • Fiber, seventeen amino acids.
  • Minerals: such as potassium, sulfur, zinc and phosphorus and lesser quantities of calcium, manganese, selenium, iron and magnesium, plus low levels of sodium.
  • Vitamins A, C and B-complex in low levels.
  • Antioxidants such as polyphenols (apigenine, luteoline), flavanols, flavonoids.
  • Other compounds like tannins, saponins, polysaccharides, essential oils (linalool, citral, and geraniol).
  • Sulfur containing bioactive compounds: alliin, allicin, ajoene, SAMC, etc.

Bioavailability and Metabolism

All these compounds are found in intact garlic cloves, but once they are chopped or crushed, they begin to react, and especially the sulfur-containing ones.

Aliin (S-allylcysteine sulfoxide) quickly transforms into allicin (diallyl thiosulfinate), which then turns into allyl-mercaptan. Reaction with cystine found in protein based foods forms S-allylmercaptocysteine (SAMC).

Allicin isn't very soluble in water but can transform into oil-soluble polysulfides such as diallyl disulfide (DADS), diallyl sulfide (DAS) and diallyl trisulfide (DATS).

Aged garlic extract, or AGE, transforms the harsh and strong smelling compounds of fresh garlic into stable sulfur compounds such as water-soluble compounds like SAMC and S-allyl cysteine (SAC) that seems to play a key role in the biological effects of garlic.

Antioxidant properties of Garlic

Its antioxidant properties derived from its phenolic compounds may help reduce inflamation and lower cardiovascular risk in obese subjects. It appears to improve the activity of immune cells and has been tested as a treatment for colds and flu (2). There’s not enough evidence to show whether garlic is helpful for the common cold (5).

A statistical review of clinical trials shows that "Garlic supplementation increased serum level of total antioxidant capacity (TAC) and superoxide dismutase (SOD) while it reduced the malondialdehyde serum level (MDA) ... Garlic supplementation may be useful to reduce oxidative stress and related diseases." Increased TAC and SOD mean that there are more antioxidants in the blood, and a lower MDA level means that the body is producing less oxidizing free radicals (that lead to an overproduction of MDA) (6).

Garlic's Lipid lowering effects (cholesterol and triglycerides)

The evidence is conflicting regarding garlic's effects on blood lipids. Some studies suggest that garlic supplements may reduce total cholesterol and the "bad" LDL or low-density lipoprotein cholesterol levels in patients with blood lipids levels. But the effect appears to be small.
Regarding triglycerides and the "good" cholesterol HDL or high-density lipoprotein (HDL) there is no effect whatsoever (5).

Garlic and Cancer

Clinical studies have not yet shown any evidence linking garlic intake with a lower risk of gastric, breast, lung, or endometrial cancer.

Though very limited, "there is credible evidence for an association between garlic intake and colon, prostate, esophageal, larynx, oral, ovary, and renal cell cancers" (6)
Some epidemiologic studies suggest a link between higher intakes of garlic and other related vegetables such as onions, shallots, chives, and leeks and a lower risk for gastrointestinal cancers (5).

Six studies do not show that intake of garlic reduces the risk of colon cancer, but 3 weaker and more limited studies suggest that garlic intake may reduce this risk. On the basis of these studies, it is highly unlikely that garlic intake reduces the risk of colon cancer.
Three studies do not show that intake of garlic reduces the risk of prostate cancer, but one weaker, more limited study suggests that intake of garlic may reduce this risk. On the basis of these studies, it is highly uncertain whether garlic intake reduces the risk of prostate cancer.
One small study suggests that garlic intake may reduce the risk of esophageal, larynx, oral, ovary, and renal cell cancers. However, the existence of such a relation between garlic intake and these cancers is highly uncertain. Kim and Kwon (2009) (3)

Garlic & High blood pressure

Blood pressure Garlic supplements may be helpful for high blood pressure, but the evidence is limited (5).

A meta‐analysis done in 2015 suggested that garlic supplements are superior to the controls ⁄ placebo in lowering blood pressure, especially in patients with high blood pressure (7).

Diabetes and Garlic

Similar to blood lipids, a meta-analysis of clinical trials suggests that "Garlic can reduce lipid profile as well as glucose parameters and be therapeutically effective in patients suffering from cardiovascular diseases and diabetes" (8).

Garlic for Skin diseases

Anti-fungal properties

Garlic extracts have a fungicidal activity against several fungi such as Trichophyton, Candida, Aspergillus, Rhodotorula and Meyerozyma.


Its oil is "applied externally to cure ringworm, warts, and skin parasites" (4).

Garlic extract applied "two times daily for four weeks had greater potential on patients with recalcitrant multiple common warts compared to any other treatment"(2).

Venous ulcers

A pilot study treated venous ulcer patients with topical herbal ointment that contained garlic and after 7 weeks it showed "anti-erythematous, epithelizing, and anti-edematous properties and decreased the venous ulcer area (2).

Liver Health and fatty liver

Nonalcoholic fatty liver disease or NAFLD affects almost 1 in 5 adults across the globe and is characterized by a build-up of fat in the liver, it is observed in people who have metabolic syndrome, are overweight, obese or have insulin resistance. Garlic seems to offer some alternative treatment for them.

Garlic supplements reduce the liver enzyme aspartate transaminase levels (AST) a sign of liver health improvement (2).

A randomized clinical trial involving 110 subjects found that 51.1% of those taking garlic improved in hepatic steatosis (fatty liver) compared to only 15.7% of the patients in the placebo group. Garlic improved liver enzymes AST, alanine transaminase (ALT) and markers such as blood sugar, total cholesterol, triglycerides and LDL cholesterol in comparison to the placebo. The authors concluded that "the intake of garlic powder was accompanied by a significant improvement in the hepatic steatosis and comorbidity related to this condition among subjects with NAFLD" (9).

Garlic: Safety and risks

Safe Daily doses of Garlic

The daily dose of garlic for adults recommended by the World Health Organization (WHO) is the following:

Garlic Product

Daily dose

Fresh raw garlic

2-5 g

Dry garlic powder

0.4-1.2 g

Garlic oil

2-5 mg

Garlic extract (Solid)

0.3-1 g

AGE (liquid)

2.4 g

Toxicity and Interactions with other medicines

Although the U.S. Food and Drug Administration considers garlic as "Generally regarded as safe (GRAS)" as a spice and food flavoring ingredient. There is evidence that garlic can be toxic if consumed in large amounts.

Aliicin if consumed in large doses can harm the liver and some sulfur compounds found in essential oils may cause nausea, vomiting, stomach ulcers, burning in mouth and throat, intestinal damage, eyesight loss, bleeding gums and damage to the mucosa.

Like any herbal or plant based food, garlic can interact with conventional medicines and other complementary health herbal supplements or ingredients. Studies have showed that it may pose a risk to people taking drugs to lower blood pressure (antihypertensive), for blood thinning (anticoagulant), HIV treatment (saquinavir), or cholesterol lowering (hypoglycemic). It also affects hypoglycemic medications increases the risk of hypoglycemia. It may interact with anticoagulants (heparin, warfarin, and aspirin) to increase the risk of bleeding and interact negatively with anesthesia. Tell your physician if you are planning on taking garlic -or any other plant based product- as a supplement (4).

It isn't clear if its safe to use garlic supplements or apply garlic to the skin during pregnancy or while breastfeeding (5).

Other Side effects include strong garlic smell in breath and body odor especially after eating raw garlic. Some people have allergic reactions to garlic.

References and Further Reading

(1) Discorides, De materia medica, also see this Greek text and facsimile

(2) Ansary J, et al., (2020). Potential Health Benefit of Garlic Based on Human Intervention Studies: A Brief Overview. Antioxidants (Basel). 2020 Jul 15;9(7):619. doi: 10.3390/antiox9070619. PMID: 32679751

(3) Kim Ji Yeon and Kwon Oran, (2009). Garlic intake and cancer risk: an analysis using the Food and Drug Administration’s evidence-based review system for the scientific evaluation of health claims. The American Journal of Clinical Nutrition, Vol 89:1 p257-264 DOI:

(4) Verma T, Aggarwal A, Dey P, Chauhan AK, Rashid S, Chen KT, Sharma R. (2005). Medicinal and therapeutic properties of garlic, garlic essential oil, and garlic-based snack food: An updated review. Front Nutr. 2023 Feb 16;10:1120377. doi: 10.3389/fnut.2023.1120377. PMID: 36875845

(5) Garlic. National Center for Complementary and Integrative Health (NCCIH). Last Updated: December 2020, accessed 11 Oct. 2023

(6) Askari M, et al., (2021). Effects of garlic supplementation on oxidative stress and antioxidative capacity biomarkers: A systematic review and meta-analysis of randomized controlled trials. Phytother Res. 2021 Jun;35(6):3032-3045. doi: 10.1002/ptr.7021. Epub 2021 Jan 23. PMID: 33484037

(7) Wang HP, Yang J, Qin LQ, Yang XJ. (2015). Effect of garlic on blood pressure: a meta-analysis. J Clin Hypertens (Greenwich). 2015 Mar;17(3):223-31. doi: 10.1111/jch.12473. Epub 2015 Jan 5. PMID: 25557383

(8) Shabani E, Sayemiri K, Mohammadpour M. (2018). The effect of garlic on lipid profile and glucose parameters in diabetic patients: A systematic review and meta-analysi. Prim Care Diabetes. 2019 Feb;13(1):28-42. doi: 10.1016/j.pcd.2018.07.007. Epub 2018 Jul 23. PMID: 30049636

(9) Soleimani D, Paknahad Z, Rouhani MH. (2020). Therapeutic Effects of Garlic on Hepatic Steatosis in Nonalcoholic Fatty Liver Disease Patients: A Randomized Clinical Trial. Diabetes Metab Syndr Obes. 2020 Jul 7;13:2389-2397. doi: 10.2147/DMSO.S254555. PMID: 32753923

About this Article

Garlic and its health benefits, A. Whittall

©2023, 11 Oct. 2023. Update scheduled for 11 Oct. 2025.

Tags: garlic, heart disease, diabetes, cholesterol, cancer, blood pressure, liver, warts

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