Overview

Garlic, onions, leeks, shallots, and scallions (the Allium genus) have been used medicinally for thousands of years across Egyptian, Greek, and Chinese traditions, and remain among the most-studied culinary plants for cardiovascular and immune-related effects.

Active compounds and evidence

Alliums contain the odorless compound alliin, which is converted by the enzyme alliinase into allicin — a pungent, chemically unstable compound — only when the cell structure is disrupted by cutting, crushing, or chewing. Allicin breaks down further into other organosulfur compounds such as ajoene, diallyl sulfide, and diallyl disulfide. Heat destroys alliinase, so raw or minimally cooked garlic yields more allicin than garlic added early in cooking.

Garlic (Allium sativum): allicin and its breakdown products have documented antimicrobial and antithrombotic (platelet-aggregation-reducing) activity in laboratory studies. Clinical trials on blood pressure show modest reductions, in some studies comparable in magnitude to low-dose medication, though study quality varies. Letting crushed garlic sit for about 10 minutes before cooking is a commonly cited technique to maximize allicin formation before heat is applied. Aged garlic extract (e.g., Kyolic) is a differently-processed, odorless form studied specifically for cardiovascular outcomes, relying on stable compounds like S-allyl cysteine rather than allicin.
Onions: the flavonol quercetin is concentrated in onions, especially red and yellow varieties, and has antioxidant and antihistamine properties studied in vitro and in some human trials. Onions also contain prebiotic inulin, supporting gut bacteria.
Immune effects: allicin has shown activity against macrophages and natural killer cells in laboratory studies, lending some biological plausibility to garlic’s traditional use for colds; human trial evidence for preventing (not curing) upper respiratory infections is modest but present.

Preparation notes

Crushing, mincing, or thinly slicing increases surface area and allicin formation; letting garlic rest about 10 minutes after cutting before cooking preserves more of the active compound. Roasting whole garlic mellows the flavor and reduces sulfur compound content. Black garlic, made by slow fermentation, has a sweet, balsamic flavor, no allicin, but retains other antioxidant compounds like S-allyl cysteine.

Safety and contraindications

Blood thinners: Garlic’s antiplatelet effect means caution is warranted with warfarin, aspirin, or other anticoagulant/antiplatelet medications — bleeding risk may be increased, and surgeons commonly recommend stopping high-dose garlic supplements roughly two weeks before surgery.
GERD: Garlic and onions can trigger reflux symptoms in sensitive individuals.
FODMAP sensitivity: Both garlic and onion are high-FODMAP foods and are common triggers for IBS symptoms.
Food-level culinary use is safe for the general population; concentrated supplements warrant more caution given the above interactions.

Sources

Clinically reviewed by Dr. Shilpa Thakur. Last reviewed 2026-07-20. Educational information, not medical advice; consult a qualified clinician before combining garlic supplements with anticoagulant medication or before surgery.