What is Garlic?
Garlic has a high concentration of sulfur-containing compounds. Thiosulfinates, which include allicin, are the main active components in garlic. It also contains:
- High levels of saponins, phosphorus, potassium, sulfur, and zinc
- Moderate levels of selenium and vitamins A and C
- Low levels of calcium, magnesium, sodium, iron, manganese, and B-complex vitamins 
Purported Health Benefits of Garlic
Though there is a solid amount of small-scale, low-quality research about garlic, proper clinical trials are lacking. Thus, most of the purported health benefits of garlic have not been proven .
1) Boosts the Immune System
In animal and cell-based studies, aged garlic extract stimulates white blood cells (lymphocytes, macrophages, monocytes, and neutrophils) by increasing glutathione. White blood cells are immune cells that provide protection against infections, while glutathione is an antioxidant that protects immune cells from free radicals [6, 7, 8].
2) Effect on the Cold and Flu
In a study (DB-RCT) of 120 individuals, aged garlic extract reduced the severity of colds and the flu by increasing the number of immune cells (T cells and NK cells) and by boosting the immune system. Aged garlic extract increases the activity of immune cells while lowering inflammatory proteins (cytokines) [8, 9].
However, solid evidence is still lacking to determine whether garlic can help fight the cold and fly. Larger, better-designed studies are needed.
3) Yeast Infections
In petri-dish studies, fresh garlic extract inhibited the growth of Candida, the most common type of yeast infections. Allicin in garlic inhibits the growth of candida by destroying fats present in the outer surface of the yeast. We can’t draw any conclusions from cell-based studies; both animal and human studies are lacking to back up this purported health benefit [5, 10, 11].
4) May Help Prevent Tooth Decay and Oral Infections
Garlic has antibacterial effects on dental plaque bacteria that cause tooth decay if left untreated. Garlic may also help with oral infections like periodontitis, oral thrush, and sore mouth from dentures. Some researchers believe garlic may be used in conjunction with antibiotics [12, 13, 14, 15].
Allicin in garlic combats bacteria by inhibiting sulfur-containing enzymes that bacteria need for survival .
5) Possible Effects on HIV
In a cell-based study, diallyl disulfide in garlic inhibited cell growth and selectively killed HIV-infected immune cells. Diallyl disulfide also inhibits virus replication by decreasing the production of proteins involved in HIV replication .
Ajoene, a garlic extract, prevents normal blood cells from fusing with HIV-infected cells and inhibits HIV replication in infected cells. Ajoene may prevent cell fusion by inactivating platelet integrins (a protein that causes blood cells to fuse together) in the blood .
However, these findings only relate to cell-based studies. Animal studies are lacking. What’s more, we can’t determine the potential effects of garlic on HIV until proper clinical trials are carried out.
6) Ulcers Caused By H. pylori
Raw garlic has antibacterial effects against H. pylori, the most common bacterial infection in the world and the main cause of ulcers in human and animal studies. Allicin in garlic reacts with proteins resulting in the inhibition of pathways associated with inflammation (TLR4 and NF-kB) [19, 20, 21].
Garlic oil soothes ulcers in rats by increasing the concentration of antioxidant enzymes and by inhibiting proteins that cause inflammation .
Human studies are lacking to confirm this purported health benefit.
7) Intestinal Infections Caused By Parasites
Limited studies suggest garlic has the potential to help with parasitic intestinal infections like giardiasis and tapeworm infections, based on studies in rats.
Allicin in garlic disrupts the mobility, food absorption, and reproduction of the parasites by blocking fat synthesis in the parasites. Garlic also promotes immune function and strengthens the body’s defense mechanism against parasitic infections by stimulating white blood cells [23, 24].
Human studies are needed.
According to the National Center for Complementary and Integrative Health (NIH) :
“Some studies indicate that certain groups of people who eat more garlic may be less likely to develop certain cancers, such as stomach and colon cancers. However, garlic in dietary supplement form has not been shown to help reduce the risk of these cancers. The National Cancer Institute recognizes garlic as one of several vegetables with potential anticancer properties but does not recommend using garlic dietary supplements for cancer prevention.”
S-allylmercaptocysteine (SAMC), a sulfur compound in garlic, decreases the growth of cancer cells and causes the death of cancer cells in cell-based studies. SAMC binds to a protein involved in cell reproduction (tubulin) and disrupts cell growth, activating proteins (JNK1 and caspase-3) that cause tumor cell death [25, 26, 27].
8) Brain Cancer
Diallyl trisulfide, a sulfur-containing compound in garlic, decreases the size of brain tumors in mice by inhibiting the enzyme histone deacetylase (HDAC), which causes tumor cell death .
9) Esophageal Cancer
Diallyl sulfide in garlic inhibits esophagus tumor formation in rats by disrupting the energy production of NMBA, which is a chemical found in fungi-contaminated foods that can cause liver and esophagus cancer [29, 30].
10) Progression of Skin Cancer
Allyl sulfides in garlic control the growth of human skin cancer cells by causing DNA damage in cancer cells in human studies. The DNA damage in cancer cells signals the p53 protein to stop cancer cell growth and to kill cancer cells [31, 32].
11) Breast Cancer Growth
12) SLung Cancer
Diallyl trisulfide in garlic works in combination with the chemotherapy drug cisplatin to inhibit lung tumor growth in mice. Diallyl trisulfide activates pathways that cause tumor cell death and prevents tumor cell growth (p53, Bcl-2, JNK, p38, and caspase) .
13) Stomach Cancer Growth
S-allylmercaptocysteine (SAMC) in garlic suppresses the growth of stomach tumors in rats. SAMC causes tumor cell death by activating the enzymes caspase and protein kinases (MAPK and PI3K/Akt) .
14) Liver Cancer Progression
15) Colon Cancer Progression
Diallyl disulfide and S-allylmercaptocysteine (SAMC) in garlic suppress colon tumor growth by stopping cancer cell growth and increasing tumor cell death in cell and rat studies.
16) Bladder Cancer
Garlic inhibits bladder tumor growth by stimulating immune cells and detoxifying carcinogens in mice.
17) Prostate Cancer Progression
S-allyl cysteine and SAMC inhibit prostate cancer cell growth by re-activating E-cadherin, a molecule that suppresses tumor invasion, in cancer patients. A low level of E-cadherin is associated with a high number of tumors and poor prognosis in prostate cancer patients [42, 43, 44].
Skin and Hair Conditions
18) May Help with Allergies
Aged garlic extract suppressed allergic reactions in mice. Ethyl acetate in aged garlic extract may directly suppress the immune protein FceRI, which is associated with the release of inflammatory factors during allergy responses. Aged garlic extract prevents inflammation during allergic reactions by inhibiting the release of histamine [45, 22].
19) Protects the Skin from Ultraviolet Rays
Garlic protects the skin from ultraviolet (UV) radiation by stimulating immune cells in human studies .
When exposed to UV rays, the urocanic acid in the skin changes, which causes suppression of the immune system. Aged garlic extract lessens the suppression of immune cells by decreasing the concentration of urocanic acid in rats .
20) Has Anti-Aging Effects
Long-term topical treatment with garlic extract may have anti-aging effects since garlic increases the growth and lifespan of skin cells. Garlic-treated skin cells are more healthy compared to untreated cells .
21) May Help with Skin Rashes
Garlic may soothe skin rashes like psoriasis and eczema. Activation of the compound NF-kB has been linked to skin rashes. NF-kB, which is activated by free radicals, cancer-causing agents, and UV light, causes inflammation.
S-allyl cysteine in garlic suppresses the pathway of NF-kB by inhibiting free radicals and lowering oxidative stress in cell model and human studies .
22) May Help Remove Scars
Garlic helps with keloid scars, which are tough scars caused by the overgrowth of skin collagen. Garlic inhibits growth factors, nitric oxide, and enzymes involved in the production of collagen .
23) Helps with Hair Loss
Some scientists think garlic gel in combination with steroid cream might be good for patients suffering from alopecia, a type of hair loss that results from immune cells attacking hair follicles. Diallyl disulfide in garlic may prevent the autoimmune response and induce hair re-growth by increasing immune suppressor cells [46, 53, 54].
Heart Health and Metabolic Syndrome
24) Lowers Cholesterol Levels
However, the evidence about garlic’s cholesterol-lowering effect is mixed. According to the NIH, even if garlic does lower blood cholesterol, “the effect is small, and low-density lipoprotein (LDL) cholesterol (the so-called ‘bad’ cholesterol that’s linked to increased heart disease risk) may not be reduced at all.” 
25) Lowers Blood Pressure
Garlic reduces blood pressure in patients with high blood pressure (hypertension). Aged garlic extract reduces blood pressure by increasing calcium and reducing C-reactive protein levels, which cause inflammation and elevated blood pressure [60, 61, 62].
On the other hand, sulfur deficiency may play a role in hypertension. Allicin is a sulfur compound in garlic that lowers blood pressure by increasing hydrogen sulfide concentrations. Hydrogen sulfide relaxes blood vessels (through nitric oxide) and prevents blood vessel constriction (by endothelin-1) [63, 63, 64].
26) Helps Prevent Heart Disease
Heart disease is associated with high cholesterol, high blood pressure, increased platelet aggregation, and the hardening of blood vessels. Platelets stop bleeding by clotting blood vessel injuries. However, platelet aggregation also leads to blood clots, which increase the risk of heart disease.
Limited evidence suggests that garlic may lower cholesterol, reduce blood pressure, relax hardened blood vessels, and prevent platelet aggregation in patients with heart disease. This is uncertain and large-scale studies are needed [1, 65].
S-allyl cysteine in aged garlic extract inhibits enzymes involved in cholesterol production. Garlic extract also increases the production and function of nitric oxide, which relaxes blood vessels and lowers blood pressure .
27) May Help Prevent Obesity
Garlic prevents obesity by reducing body weight and fat accumulation in mice studies. In animal studies, garlic activates proteins (AMPK and uncoupling proteins) in fat tissue, liver, and muscle, which converts nutrients into heat instead of energy storage [69, 70, 71].
Ajoene, a compound found in garlic, prevents obesity by decreasing fat tissue in rats. Ajoene generates hydrogen peroxide, which activates enzymes (protein kinases) that kill fat cells .
28) May Help with Diabetes
Diabetes is caused by genetics, obesity, high cholesterol, blood pressure, or blood glucose. Insulin resistance occurs when the body no longer responds to insulin, leading to increased blood sugar levels and a high risk of developing diabetes.
Garlic reduced blood sugar levels in rats by decreasing the activity of enzymes (phosphatases and aminotransferases) involved in the transportation of glucose in the liver, a sugar that is the body’s main source of energy .
Detoxifies the Body
29) Helps Detoxification By the Liver
Toxins like pesticides, environmental pollutants, and chemicals cause oxidative stress and inflammation in the body. Garlic detoxifies the body through its antioxidant effects. Sulfur-containing compounds in garlic decrease oxidative stress and reduce inflammation in rats .
Garlic detoxifies the liver by increasing the activity of detoxifying enzymes glutathione S-transferase (GST) and CYP2B. Garlic also inhibits CYP2E1 enzymes, which produce free radicals and cause oxidative stress-induced damage and inflammation [76, 36].
30) Helps Damage Caused By Liver Toxicity
Garlic powder protects against damage from liver toxicity caused by high doses of antibiotics, Tylenol, and lead in rat studies. Garlic acts as an antioxidant and prevents oxidative stress by stabilizing free radicals. The decrease in oxidative stress may increase the activity of antioxidant enzymes, which prevents further damage to the liver [78, 79, 80, 25].
31) Helps Damage Caused By Kidney Toxicity
Garlic helped with kidney failure caused by the antibiotic gentamicin and kidney damage caused by cisplatin, a chemotherapy drug, in rats .
S-allyl cysteine, a sulfur compound in garlic, acts as an antioxidant by inhibiting free radicals, which cause cellular damage in the body. By lowering free radicals, it increases the activity of antioxidant enzymes in the kidney. S-allyl cysteine also inhibits the enzymes that produce free radicals .
32) Potential Effects on the Aging Brain & Brain Diseases
Free radicals are highly unstable molecules that are formed when oxygen is used in the body to produce energy. Bodily antioxidants inhibit free radicals by stabilizing the molecules. Without enough antioxidants, free radicals cause cellular damage in the body .
S-allyl cysteine is an antioxidant found in aged garlic extract that protects against brain damage in a cell model study. S-allyl cysteine activates antioxidant enzymes in the brain (hippocampus) that decreases free radicals preventing damage .
Clinical studies are lacking.
33) May Improve Memory
Side Effects and Precautions
Although garlic consumption is safe, ingested garlic can cause bad breath and body odor. Consuming an excessive amount of raw garlic, especially on an empty stomach, may lead to an upset stomach, gas, and changes in intestinal bacteria [87, 88].
Due to its anti-blood-clotting abilities, a high dose of garlic has an interaction with blood thinners like aspirin and warfarin. Garlic supplements should be stopped seven days before surgical procedures to prevent any complications .
Mutations in the transient receptor potential V1 (TRPV1) gene may cause hypersensitivity to allicin in garlic extract in humans. The TRPV1 gene is found throughout the nervous system, bladder, tongue, and skin.
The mutations in the gene might cause structural changes in the TRPA1 protein, making it more sensitive to allicin. The TRPA1 protein is an allicin receptor that triggers inflammation and a pain response when activated [92, 93].
Aged black garlic is a garlic preparation with a sweet and sour taste and no strong odor.
Aged black garlic is produced by aging garlic at high temperature for two to three weeks. It contains high levels of organic sulfur compounds like water-soluble S-allyl cysteine and polyphenols.
Dosage and Supplements
- Garlic oil – Water-soluble compounds and allicin are eliminated by this process. It contains a variety of sulfides, including diallyl disulfide
- Garlic oil macerate – Made of encapsulated garlic cloves ground into vegetable oil. Oil macerate contains allicin, which decomposes quickly into other compounds (dithiins, ajoene, and sulfides)
- Garlic powder – Dried and pulverized into powder. The main sulfur compound in garlic powder is alliin. Garlic powder does not contain allicin
- Aged garlic extract – Aged raw garlic has an increased concentration of antioxidant compounds. Allicin decomposes into other compounds, including S-allyl cysteine, which is one of the most active compounds in aged garlic extract
For adults, the recommended amount is 4 grams (1 to 2 cloves) of raw garlic, 300 mg dried garlic powder tablet 2 to 3 times or 7.2 grams of garlic extract per day .
Alliin, an amino acid containing sulfur, is broken down by the enzyme alliinase and converted to allicin when raw garlic is chopped or crushed. Allicin is an unstable compound that degrades quickly, so be sure to chop your garlic right before use .