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Medically reviewed on September 17, 2018

Scientific Name(s): Allium cepa L. Family: Liliaceae (lilies).

Common Name(s): Onion , Bulbus Allii Cepae , common onion , garden onion . Topical commercial preparations include Contractubex and Mederma .


Onion has potential in treating cardiovascular disease, hyperglycemia, and stomach cancer, although few quality clinical trials are available to support these uses. Topical preparations have been evaluated for the prevention of surgical scarring with varying results.


Clinical trials are lacking to provide dosage recommendations. Average daily doses of 50 g of fresh onion, 50 g of fresh onion juice, or 20 g of dried onion have been suggested. Topical onion extract gels have been used in studies evaluating the effect on scarring and are generally applied 3 times daily.


Contraindications have not been identified.


Generally recognized as safe when used as food. Avoid dosages above those found in foods because safety is unproven.


None well documented.

Adverse Reactions

Few reported.


Information is limited.


The onion plant is a perennial herb growing to about 1.2 m in height, with 4 to 6 hollow, cylindrical leaves. On top of the long stalks, greenish-white flowers are present in the form of solitary umbels growing to 2.5 cm wide. The seeds of the plant are black and angular. The underground bulb, which is used medicinally, is comprised of fleshy leaf sheaths forming a thin-skinned capsule, and varies greatly in size (2 to 20 cm). Shape (flattened, spherical, or pear-shaped) and color depend on the variety. 1 , 2 , 3


The onion is one of the world's leading vegetable crops, believed to have been domesticated in central and western Asia. Onions were used as early as 5,000 years ago in Egypt, as depicted on ancient monuments; ancient Greek and Roman records also make references to the onion. Onions were consumed throughout Europe during the Middle Ages and were later thought to guard against evil spirits and the plague, probably because of their strong odor. Folk healers traditionally used onions to prevent infections, and an onion and garlic concoction cooked in milk was used as a European folk remedy for congestion. Onion skin dye has also been used for egg and cloth coloring in the Middle East and Europe. Christopher Columbus is said to have introduced the plant to North America on his 1492 expedition. Onions are routinely used in homeopathic medicine. 2 , 3 , 4


Onions contain 89% water, 1.5% protein, and vitamins B 1 , B 2 , and C, along with potassium and selenium. Polysaccharides such as fructosans, saccharose, and others are also present, as are peptides, flavonoids (mostly quercetin), and essential oil. Methods for the qualitative assessment of the flavonoids have been detailed, and quercetin glycosides have been shown to be heat-stable and transferable to cooking water.

Onion contains numerous sulfur compounds, including thiosulfinates and thiosulfonates; cepaenes; S-oxides; S,S-dioxides; mono-, di-, and tri-sulfides; and sulfoxides. Mincing or crushing the bulb releases cysteine sulfoxide from cellular compartments, making contact with the enzyme alliinase from the adjacent vacuoles. Hydrolysis results with the release of reactive intermediate sulfenic acid compounds and then to the various sulfur compounds. 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9

Uses and Pharmacology


Large epidemiological studies infer a protective effect of onion (as well as garlic and leek) consumption against stomach and colorectal cancers. 5 , 10 , 11 Case-control (retrospective) data also suggest an inverse relationship between onion consumption and benign prostatic hypertrophy and endometrial cancer. 12 , 13 An overestimation of effect for these 2 cancers is possible due to publication and recall bias, and insufficient evidence exists for breast, lung, and other cancers. 5 , 10 , 11 , 14 In the case of GI-related cancers, proposed mechanisms of action for the Allium species include an inhibition of Helicobacter pylori and other bacterial activity, as well as a general decrease in the endogenous production of carcinogenic N-nitroso compounds. 10 , 11 In vitro and animal model studies describe the inhibition of mutagenesis, modulation of enzyme and cell signaling pathways, free-radical scavenging, apoptosis, and other effects on cell proliferation and tumor growth. 5 , 10 , 15 , 16 , 17 , 18 , 19 Prospective clinical trials are lacking.

Cardiovascular disease

Certain onion genotypes containing higher contents of sulfur in the bulb correlate with greater antiplatelet activity. Thiosulfinates dimethyl- and diphenyl-thiosulfinate, for example, are known to slow thrombocyte biosynthesis. The least polar fraction of onion extract was associated with the most inhibitory activity toward platelet aggregation; thus, a greater inhibition of thromboxane synthesis was reported. 11 , 20 , 21

Animal data

Hypolipidemic effects of onion sulfur-compounds, including S-methyl cysteine sulfoxide and allylpropyl disulfide, have been demonstrated in rats and rabbits, and include effect on diet-induced atherosclerosis, hypolipidemic action, and inhibitory effects on platelet formation. Raw, but not cooked, onion demonstrated antithrombotic effects in rats. 22 , 23 , 24 , 25 , 26

Clinical data

Quercetin, onion extract, and onion soup have been evaluated in vitro and in vivo for effect on platelet aggregation in limited studies with mixed results. 2 , 11 , 27 , 28 Quercetin, but not raw onion, decreased blood pressure in hypertensive subjects in one study, 29 and mixed results have been found for effects on hyperlipidemia. 5 , 30 , 31 Trials have generally been conducted with small populations, and evidence is lacking to support claims of cardiovascular benefits.


Limited clinical trials have been conducted on the effects of onion extract gels on scarring, with equivocal results. Inhibition of human skin fibroblasts and keloidal fibroblasts has been demonstrated in vitro. 2 In certain populations, a reduction in the effects of surgical procedures, such as redness, itching, burning, and pain, has been demonstrated, 32 , 33 , 34 , 35 while in others there was no difference over standard petroleum-based emollient therapy. 33 , 36


Hypoglycemic effects of onion extracts have been demonstrated in animal models. 2 , 37 , 38 , 39 Quality clinical trials are lacking; however, limited trials have shown a decrease in blood glucose levels in healthy volunteers and non-insulin–dependent diabetic subjects with aqueous onion extract and raw dietary onion. 2 , 11

Other uses
Antiallergy/Respiratory effects

Compounds derived from onion have exerted anti-inflammatory and antihistamine effects in vitro and in animal models. 2 , 11

Antimicrobial effects

In vitro studies have shown onion to possess antibacterial (including H. pylori ), antiparasitic, and antifungal activity. 1 , 5 , 40 , 41 Clinical applications for this activity have not been determined, and use as a food preservative is limited by the strong odor and instability of sulfur compounds. 11


Clinical trials are lacking to provide dosage recommendations. An average daily dose of 50 g of fresh onion, 50 g of fresh onion juice, or 20 g of dried onion have been suggested. 2 , 3 Topical onion extract gels have been used in studies evaluating the effect on scarring, and are generally applied 3 times daily. 33 , 36


Generally recognized as safe when used as food. Avoid dosages above those found in foods because safety and efficacy are unproven.


None well documented. No potentiation of antiplatelet effects has been reported.

Adverse Reactions

Ingestion of onion and onion extract appears to be relatively safe. 2 , 3 Onion seeds have been reported as occupational allergens, but frequent contact with onion rarely causes allergic reaction. 42 Certain sulfur compounds, such as propanethial-S-oxide, escape from onion in vapor form and hydrolyze to sulfuric acid when cut, causing eye irritation and lacrimation. 43 Using a sharp knife minimizes the crushing of onion tissue and liberation of volatiles, and cutting an onion under running water reduces lacrimation.


Mutagenicity of Bulbus Allii Cepae was not demonstrated in vitro. 2 Low doses of onion (50 mg/kg) given to rats had little effect on lung and liver tissues, but high doses (500 mg/kg) resulted in histological changes in these organs. Intraperitoneal administration was more damaging than oral administration, resulting in 25% mortality in rats. 44 Poisoning in cattle fed large quantities of onion has been reported. 45 Food colorant extracted from the tan onion bulb covering had no acute or subacute toxic effects in mice. 46


1. Allium cepa L. USDA, NRCS. 2007. The PLANTS Database ( , July 2010). National Plant Data Center, Baton Rouge, LA 70874-4490 USA.
2. Bulbus Allii Cepae. In: WHO Monographs on Selected Medicinal Plants . Vol. 1. Geneva, Switzerland: World Health Organization; 1999: 5.
3. Blumenthal M, Busse WR, Goldberg A, eds, et al. The Complete German Commission E Monographs . Austin, TX: American Botanical Council; 1998:176-177.
4. Lanzotti V. The analysis of onion and garlic. J Chromatogr A . 2006;1112(1-2):3-22.
5. Rose P, Whiteman M, Moore PK, Zhu YZ. Bioactive S-alk(en)yl cysteine sulfoxide metabolites in the genus Allium : the chemistry of potential therapeutic agents. Nat Prod Rep . 2005;22(3):351-368.
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7. Wiczkowski W, Romaszko J, Bucinski A, et al. Quercetin from shallots ( Allium cepa L. var. aggregatum ) is more bioavailable than its glucosides. J Nutr . 2008;138(5):885-888.
8. Nemeth K, Piskula MK. Food content, processing, absorption and metabolism of onion flavonoids. Crit Rev Food Sci Nutr . 2007;47(4):397-409.
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12. Galeone C, Pelucchi C, Dal Maso L, et al. Allium vegetables intake and endometrial cancer risk. Public Health Nutr . 2009;12(9):1576-1579.
13. Galeone C, Pelucchi C, Talamini R, et al. Onion and garlic intake and the odds of benign prostatic hyperplasia. Urology . 2007;70(4):672-676.
14. Challier B, Perarnau JM, Viel JF. Garlic, onion and cereal fibre as protective factors for breast cancer: a French case-control study. Eur J Epidemiol . 1998;14(8):737-747.
15. Munday R, Munday CM. Induction of phase II enzymes by aliphatic sulfides derived from garlic and onions: an overview. Methods Enzymol . 2004;382:449-456.
16. Herman-Antosiewicz A, Singh SV. Signal transduction pathways leading to cell cycle arrest and apoptosis induction in cancer cells by Allium vegetable-derived organosulfur compounds: a review. Mutat Res . 2004;555(1-2):121-131.
17. Boyle SP, Dobson VL, Duthie SJ, Kyle JA, Collins AR. Absorption and DNA protective effects of flavonoid glycosides from an onion meal. Eur J Nutr . 2000;39(5):213-223.
18. Fukushima S, Takada N, Hori T, Wanibuchi H. Cancer prevention by organosulfur compounds from garlic and onion. J Cell Biochem Suppl . 1997;27:100-105.
19. Izawa H, Kohara M, Aizawa K, et al. Alleviative effects of quercetin and onion on male reproductive toxicity induced by diesel exhaust particles. Biosci Biotechnol Biochem . 2008;72(5):1235-1241.
20. Goldman IL, Kopelberg M, Debaene JE, Schwartz BS. Antiplatelet activity in onion ( Allium cepa ) is sulfur dependent. Thromb Haemost . 1996;76(3):450-452.
21. Miller LG, Murray WJ, eds. Herbal Medicinals, A Clinician's Guide . New York: Pharmaceutical Products Press; 1998:195-202.
22. Wilcox BF, Joseph PK, Augusti KT. Effects of allylpropyl disulphide isolated from Allium cepa Linn on high-fat fed rats. Indian J Biochem Biophys . 1984;21(3):214-216.
23. Lata S, Saxena KK, Bhasin V, Saxena RS, Kumar A, Srivastava VK. Beneficial effects of Allium sativum , Allium cepa and Commiphora mukul on experimental hyperlipidemia and atherosclerosis—a comparative evaluation. J Postgrad Med . 1991;37(3):132-135.
24. Kumari K, Mathew BC, Augusti KT. Antidiabetic and hypolipidemic effects of S-methyl cysteine sulfoxide isolated from Allium cepa Linn. Indian J Biochem Biophys . 1995;32(1):49-54.
25. Bordia T, Mohammed N, Thomson M, Ali M. An evaluation of garlic and onion as antithrombotic agents. Prostaglandins Leukot Essent Fatty Acids . 1996;54(3):183-186.
26. Chen JH, Chen HI, Tsai SJ, Jen CJ. Chronic consumption of raw but not boiled Welsh onion juice inhibits rat platelet function. J Nutr . 2000;130(1):34-37.
27. Hubbard GP, Wolffram S, de Vos R, Bovy A, Gibbins JM, Lovegrove JA. Ingestion of onion soup high in quercetin inhibits platelet aggregation and essential components of the collagen-stimulated platelet activation pathway in man: a pilot study. Br J Nutr . 2006;96(3):482-488.
28. Hubbard GP, Wolffram S, Lovegrove JA, Gibbins JM. Ingestion of quercetin inhibits platelet aggregation and essential components of the collagen-stimulated platelet activation pathway in humans. J Thromb Haemost . 2004;2(12):2138-2145.
29. Edwards RL, Lyon T, Litwin SE, Rabovsky A, Symons JD, Jalili T. Quercetin reduces blood pressure in hypertensive subjects. J Nutr . 2007;137(11):2405-2411.
30. O'Reilly JD, Mallet AI, McAnlis GT, et al. Consumption of flavonoids in onions and black tea: lack of effect on F2-isoprostanes and autoantibodies to oxidized LDL in healthy humans. Am J Clin Nutr . 2001;73(6):1040-1044.
31. McAnlis GT, McEneny J, Pearce J, Young IS. Absorption and antioxidant effects of quercetin from onions, in man. Eur J Clin Nutr . 1999;53(2):92-96.
32. Ho WS, Ying SY, Chan PC, Chan HH. Use of onion extract, heparin, allantoin gel in prevention of scarring in chinese patients having laser removal of tattoos: a prospective randomized controlled trial. Dermatol Surg . 2006;32(7):891-896.
33. Koc E, Arca E, Surucu B, Kurumlu Z. An open, randomized, controlled, comparative study of the combined effect of intralesional triamcinolone acetonide and onion extract gel and intralesional triamcinolone acetonide alone in the treatment of hypertrophic scars and keloids. Dermatol Surg . 2008;34(11):1507-1514.
34. Draelos ZD. The ability of onion extract gel to improve the cosmetic appearance of postsurgical scars. J Cosmet Dermatol . 2008;7(2):101-104.
35. Jackson BA, Shelton AJ. Pilot study evaluating topical onion extract as treatment for postsurgical scars. Dermatol Surg . 1999;25(4):267-269.
36. Chung VQ, Kelley L, Marra D, Jiang SB. Onion extract gel versus petrolatum emollient on new surgical scars: prospective double-blinded study. Dermatol Surg . 2006;32(2):193-197.
37. Kook S, Kim GH, Choi K. The antidiabetic effect of onion and garlic in experimental diabetic rats: meta-analysis. J Med Food . 2009;12(3):552-560.
38. Roman-Ramos R, Flores-Saenz JL, Alarcon-Aguilar FJ. Anti-hyperglycemic effect of some edible plants. J Ethnopharmacol . 1995;48(1):25-32.
39. Sheela CG, Kumud K, Augusti KT. Anti-diabetic effects of onion and garlic sulfoxide amino acids in rats. Planta Med . 1995;61(4):356-357.
40. Elnima EI, Ahmed SA, Mekkawi AG, Mossa JS. The antimicrobial activity of garlic and onion extracts. Pharmazie . 1983;38(11):747-748.
41. Zohri AN, Abdel-Gawad K, Saber S. Antibacterial, antidermatophytic and antitoxigenic activities of onion ( Allium cepa L.) oil. Microbial Res . 1995;150(2):167-172.
42. Navarro JA, del Pozo MD, Gastaminza G, Moneo I, Audícana MT, Fernández de Corres L. Allium cepa seeds: a new occupational allergen. J Allergy Clin Immunol . 1995;96(5 pt 1):690-693.
43. Chan RS, Mandell RB. Corneal swelling caused by Allium cepa . Am J Optom Arch Am Acad Optom . 1972;49(9):713-715.
44. Thomson M, Alnaqeeb MA, Bordia T, Al-Hassan JM, Afzal M, Ali M. Effects of aqueous extract of onion on the liver and lung of rats. J Ethnopharmacol . 1998;61(2):91-99.
45. Verhoeff J, Hajer R, van den Ingh TS. Onion poisoning of young cattle. Vet Rec . 1985;117(19):497-498.
46. Kojima T, Tanaka T, Mori H, Kato Y, Nakamura M. Acute and subacute toxicity tests of onion coat, natural colorant extracted from onion ( Allium cepa L.), in (C57BL/6 × C3H)F1 mice. J Toxicol Environ Health . 1993;38(1):89-101.

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