Garlic (Allium sativum L.) (December 2008)

By Isidro C Sia, MD*

*Professor of Pharmacology, College of Medicine, University of the Philippines-Manila
First published on the RDU Update –DOH, Vol. 15 No. 1, Manila, Philippines, 2008

What do patients know about garlic

Garlic has been used both as spice and medicine for thousands of years. With the present craze on use of natural products, health maintenance or as therapeutic agents, garlic, in its various preparations, has been popularly used for high blood pressure, for high cholesterol, and for heart disease. It is also used to prevent certain types of cancer such as stomach and colon cancers.

How patients use garlic

Our people, especially those in the rural areas, eat raw garlic cloves when they feel their blood pressure has increased. Sometimes, pickled garlic cloves are used. Others would take the cloves with pineapple juice or other sour drinks for ‘added’ effect. Various preparations of garlic are available in the market. These include tablets or capsules made from the dried and powdered cloves. Oils and liquid extracts are also available.


What is scientifically known about garlic

Effect on cholesterol level


• Some evidence indicates that taking garlic can slightly lower blood cholesterol levels. Studies have shown positive effects for short-term use – 1 to 3 months.

• However, a study funded by the US National Center for Complementary and Alternative Medicine (NCCAM) showed that 3 garlic preparations (fresh garlic, dried powdered garlic tablets, and aged garlic extract tablets) found no effect on blood cholesterol levels.

Effect on blood pressure


• Current evidence is mixed on whether taking garlic can lower blood pressure, even slightly.

Effect on arteries and heart

• Preliminary research suggests that taking garlic may slow the development of atherosclerosis, a condition that can lead to heart disease or stroke.

Prevention of cancer

• Some studies suggest consuming garlic as a regular part of the diet may lower the risk of certain cancers. However, no clinical trials have proven this. A clinical trial on the long term use of garlic supplements to prevent stomach cancer found no positive effect.

Safety of garlic

• Garlic has been found to interfere with the effectiveness of the antiHIV drug saquinavir.

• Garlic may augment the effect of warfarin.


What we could counsel our patients about the use of garlic

On the safety of garlic



• Garlic appears to be safe for most adults.

• Side effects include breath and body odor, heartburn, upset stomach, and allergic

reactions. These side effects are more common with raw garlic.

• Avoid garlic in the diet or as a supplement at least one week before a surgery or dental work, if patient has bleeding disorder.

• Avoid garlic if saquinavir is being used.

For patients with hypercholesterolemia


• Studies are inconclusive on the efficacy of garlic to lower blood cholesterol.

• Counsel patient on diet, exercise, lifestyle, and the regular use of lipid-lowering medicine if indicated.


For patients with hypertension


• Studies are inconclusive on the efficacy of garlic to lower blood pressure.

• Counsel patient on diet, exercise, lifestyle, and the regular use of antihypertensive medicine if indicated.



Summary of scientific evidence on the use of garlic



High cholesterol

Multiple studies in humans have reported small reductions in total blood cholesterol and low-density lipoproteins ("bad cholesterol") over short periods of time (4 to 12 weeks). It is not clear if there are benefits after this amount of time. Effects on high-density lipoproteins ("good cholesterol") are not clear. This remains an area of controversy. Well-designed and longer studies are needed in this area.



Alopecia areata (hair loss)

Application of garlic gel on the skin may be beneficial in the treatment of alopecia areata (hair loss). Additional study is needed.




Anti-fungal (applied to the skin)

Several studies describe the application of garlic to the skin to treat fungal infections, including yeast infections. Take caution as garlic can cause severe burns and rash when applied to the skin of sensitive individuals.



Anti-platelet effects (blood thinning)

The effects of garlic on platelet aggregation have been assessed in several human trials. Because garlic has been associated with several cases of bleeding, therapy should be applied with caution (particularly in patients using other agents that may precipitate bleeding).



Atherosclerosis ("hardening" of the arteries)

Preliminary research in humans suggests that deposits of cholesterol in blood vessels may not grow as quickly in people who take garlic. It is not clear if this is due to the ability of garlic to lower cholesterol levels or to other effects of garlic.



Benign breast diseases

Taking garlic supplements by mouth may improve some symptoms of benign breast disease. Additional study is needed.




Preliminary human studies suggest that regular consumption of garlic (particularly unprocessed garlic) may reduce the risk of developing several types of cancer including gastric and colorectal malignancies. Some studies use multi-ingredient products so it is difficult to determine if garlic alone may play a beneficial role. Further well-designed human clinical trials are needed to conclude whether eating garlic or taking garlic supplements may prevent or treat cancer.



Cryptococcal meningitis

Preliminary study documented potential benefits of oral plus intravenous garlic in the management of cryptococcal meningitis. Further research is needed before recommending for or against the use of garlic in the treatment of this potentially serious condition, for which other treatments are available.




Familial hypercholesterolemia

Familial hypercholesterolemia is a genetic disorder in which very high cholesterol levels run in families. Research in children with an inherited form of high cholesterol suggests that garlic does not have a large effect on lowering cholesterol levels in these patients.



Heart attack prevention in patients with known heart disease

It is not clear if garlic prevents future heart attacks in people who have already had a heart attack. The effects of garlic on cholesterol levels may be beneficial in such patients.



High blood pressure

Numerous human studies report that garlic can lower blood pressure by a small amount, but larger, well-designed studies are needed to confirm this possible effect.



Infections (bacterial, viral, fungal, other)

In initial studies, garlic has been shown to be effective against bacteria, mycobacteria, viruses, and fungi. Small studies have been conducted using garlic for acute viral respiratory infections in children. Other studies have shown that garlic may have some effect on athlete's foot. More high-quality human studies are necessary to recommend garlic to treat or prevent infections.



Oral candidiasis

There is not enough evidence to suggest garlic is effective in the treatment of oral candidiasis.



Mosquito repellant

There is currently not enough evidence to suggest that garlic helps repel mosquitoes.



Peripheral vascular disease (blocked arteries in the legs)

Some human studies suggest that garlic may improve circulation in the legs by a small amount, but this issue remains unclear. Better-designed studies are needed.



Pre-eclampsia (prevention)

There is not enough evidence to recommend increased garlic intake for preventing pre-eclampsia and its complications.



Sickle cell anemia

Initial evidence suggests the antioxidant activity of garlic may benefit sickle cell anemia. Further study is necessary.



Systemic sclerosis

There is insufficient evidence to recommend the use of garlic in systemic sclerosis.



Tick repellant

In early study, self-reports of tick bites were significantly less in people receiving garlic over a placebo "sugar" pill. Further well-designed study is needed to confirm these results.



Upper respiratory tract infection

Preliminary reports suggest that garlic may reduce the severity of upper respiratory tract infections. However, this has not been demonstrated in well-designed human studies.




Animal studies suggest that garlic may lower blood sugar and increase the release of insulin, but studies in humans do not confirm this effect.



Stomach ulcers caused by Helicobacter pylori bacteria

Early studies in humans show no effect of garlic on gastric or duodenal ulcers.




A – Strong scientific evidence for this use

B – Good scientific evidence for this use

C – Unclear evidence for this use

D – Fair scientific evidence against this use (it may not work)

E – Strong scientific evidence against this use (it likely does not work)



Garlic (Allium sativum L.). Drugs and Supplements, Medline Plus. National Library of Medicine. National Institute of Health, Department of Health and Human Services, United Sates of America. Date accessed: March 10, 2008.

Garlic (Allium sativum L.): evidence-based monograph. Natural Standard: the Authority on Integrative Medicine. Date accessed: March 10, 2008.

Garlic: effects on cardiovascular risks and disease, protective effects against cancer, and clinical adverse effects. Evidence Report/Technology Assessment: Number 20. Agency for Healthcare Research and Quality (AHRQ). Date accessed: March 10, 2008.

Garlic. In: Blumenthal M, Goldberg A, Brinckman J, Eds. Herbal Medicine: Expanded Commission E Monographs. Newton, MA: Lippincott Williams & Wilkins; 2000: 139-148. National Center for Complementary and Alternative Medicine (NCCAM). National Institute of Health, Department of Health and Human Services, United Sates of America. Date accessed: March 10, 2008. Office of Dietary Supplements. National Institutes of Health, Department of Health and Human Services, United States of America. Date accessed: March 10, 2008.