Garlic contains 65% water, 28% carbohydrates, 2.3% organosulphur compounds, 2% protein, 1.2% free amino acids, 1.5% fiber, 0.15% lipid and trace quantities of phytic acid (0.08%), saponins (0.07%) and β-sitosterol. Vitamins B₁, B₂, B₃, B₅, B₆, C, and minerals calcium, iron, magnesium, manganese, phosphorous, potassium, sodium, zinc and selenium are also present.

The organosulphur-rich essential oil of the drug is the chief active constituent and it contains organosulphur compounds—(+)-S-allyl-L-cysteine sulphoxides namely alliin (1%), methiin (0.12%), isoalliin (0.06%) and cyloalliin (0.1%) and -alkyl-L-cysteines – g-glutamyl-S-trans1-propenylcysteine (0.6%), g-L-glutamyl-S-alylcysteine (0.4%).

Garlic bulbs contain the enzyme alliinase with which alliin comes in contact, when they are bruised or crushed. Thus releases allicin through the intermediate allyl sulphenic acid. 1 mg of alliin is equivalent to 0.45 mg of allicin. Allicin is highly unstable and in the presence of water and oxygen it gets decomposed into polysulphide, which is responsible for the unpleasant odor. Alliicin is the chief precursor of various other compounds found in garlic products such as allyl sulfides, ajoenes, and vinyl dithiins.

The various products of decomposition of allicin include diallyl disulphide, diallyltrisulphide, and the corresponding polysulphides all of which are strong-smelling compounds. (E) Ajoene and (Z) Ajoene are formed from allicin.

Aged garlic extract is prepared by incubating minced garlic in aqueous alcohol (15–20%), which is then stored for 20 months and then concentrated. This product is reported to be very low in organosulphur compound content. Its allicin content is 3% of fresh garlic.

Biological activity of garlic largely depends on its ability to produce allicin, which in turn results in the formation of other active principles.

Uses

As a medicine, garlic has been held in high esteem by ancient physicians of India. Administered in vitiated conditions of kapha and vata, it is a thermogenic stimulant and is administered in fevers, coughs, and other debilitating conditions. Externally the juice is used as a rubefacient in skin diseases and as ear drops in earache and deafness.

Garlic has been used for culinary and medicinal purposes in different regions across the world.

It has also been used to a large extent in western herbal medicine. It is used as a carminative in dyspepsia. Because of its expectorant, stimulant, and disinfectant property, garlic in the form of a juice is used in the relief of cough, colds, catarrh, and rhinitis. It is also used for anti-fertility effects and in the treatment of malignant tumors. Steam distilled oil of garlic is used as insecticide in ring worm and urinary tract infections.

Though largely used as a condiment, it is now categorized as a nutraceutical for its nutritional and medicinal properties.

It is extensively indicated in the management of hypertension and atherosclerosis. Garlic is being recommended as a preventive against age-dependent vascular changes, in mild hypertension, as a hypolipidaemic and in the prophylaxis of atherosclerosis. Several forms of garlic namely fresh bulbs, dried powder, oil macerates, juice, aqueous or alcoholic extracts, aged garlic extracts, odorless garlic products (allinase inactivated) are available for use as dietary supplements.