Garlic (Allium sativum L.) is an edible bulb that has both dietary role and medicinal properties.
It has been used traditionally to aid respiration, digestion, and treat parasitic infections.
It contains biologically active compounds, including, allicin, ajoene, and S-allylcysteine that works synergistically.
It is gaining attention in modern medicine for its numerous health benefits. 
Health Benefits of Garlic Supplements
The key benefits of garlic supplements include modulating serum lipid markers in hypercholesterolemia patients and reduce high blood pressure. It may delay the progression of atherosclerosis and the severity of a cold. It also may increase exercise tolerance, reduce physical fatigue, and prevent bone loss. Limited studies showed that it might treat lead poisoning, lower blood glucose levels and reduce the risk of Alzheimer’s disease.
1. Garlic modifies serum lipid markers in hypercholesterolemia patients.
Garlic supplements modified serum lipid markers in hypercholesterolemia patients. The lipid-modifying effect lasted for at least six months. 
In a randomized controlled trial, 600 mg of garlic treatment in mild hypercholesterolemia men decreased total cholesterol and LDL cholesterol levels.
It increased HDL cholesterol levels.   
900mg/day of garlic was well tolerated by the patients while modifying lipid markers.
Meta-analysis showed that garlic preparations slightly increased HDL cholesterol but did not affect triglyceride levels.
A combination of garlic and fish oil had a beneficial effect on serum lipid markers and was well tolerated.
What it means: Garlic supplements reduced LDL and total cholesterol levels in mild hypercholesterolemia patients. It slightly increased HDL cholesterol and did not have any effect on triglyceride levels. More well designed randomized trials are required to assess if garlic supplements can be used as a therapeutic supplement for treating hypercholesterolemia.
2. Garlic reduces high blood pressure in hypertensive individuals
Garlic supplements reduced blood pressure by counteracting oxidative stress in hypertensive individuals. 
In a study, twenty patients with essential hypertension supplemented with 250mg/day of garlic showed a reduction in blood pressure, lipid peroxidation, and nitric oxide levels.
A study showed that aged garlic supplementation reduced systolic blood pressure in patients with uncontrolled hypertension. 
Patients with stage 1 essential hypertension showed a significant decrease in systolic and diastolic blood pressure when supplemented with garlic tablets (300-1500 mg/day).
What it means: Studies showed that garlic supplements might reduce systolic and diastolic blood pressure in hypertensive individuals. Future studies should focus on determining if garlic supplements can be used as an adjunct to conventional antihypertensive therapy.
3. Garlic may delay the progression of atherosclerosis
Garlic administration may delay the progression of atherosclerosis. It may prevent coronary artery calcification and decrease arteriosclerotic plaque volume. 
In a pilot study, atherosclerotic patients were supplied with 4 ml of aged garlic extract. The treatment group had a lesser calcium score.  
A study showed that aged garlic extract plus vitamin B12, folic acid, vitamin B6, and L-arginine reduced the progression of coronary artery calcification. 
In another study involving metabolic syndrome patients, aged garlic extract reduced low-attenuation plaque %.
Continuous ingestion of high dose garlic powder showed a decrease in arteriosclerotic plaque volume.
What it means: Garlic supplements may inhibit the rate of progression of coronary calcification in atherosclerotic patients. Limited studies reported that garlic powder might have a role in decreasing arteriosclerotic plaque volume. Further, more extensive studies will determine if garlic supplements can reduce the risk of atherosclerosis.
4. Garlic may reduce the severity of cold
Studies showed that garlic supplements might reduce the severity of cold symptoms.
Volunteers taking allicin-containing garlic supplements had fewer colds. The volunteers who had cold recovered faster compared to the placebo.
In a randomized controlled trial, aged garlic extract reduced the severity of cold in healthy subjects by enhancing immune cell function. It did not reduce the incidence of cold. 
In a five-month study, 600 mg of allicor (long-lasting garlic tablets) reduced acute respiratory disease morbidity in children.
What it means: Garlic supplement may reduce the severity of the common cold and promote faster recovery. There is insufficient evidence regarding the effects of garlic in treating the common cold. Therefore, more studies are required to examine if garlic supplements can be taken for preventing or treating the common cold.
5. Garlic may increase exercise tolerance and reduce physical fatigue.
Studies reported that garlic supplementation might increase exercise tolerance and reduce exercise-induced physical fatigue.
A study involving six weeks supplementation of garlic reduced heart peak exercise and workload on heart, causing better exercise tolerance in patients with coronary artery disease.
In a mice study, aged garlic extract reduced physical fatigue caused by the endurance exercise in the treadmill. It reduced oxidative stress and promoted oxygen supply.
However, in one study, seven days of garlic supplementation to cyclists did not improve peripheral blood pressure, exercising heart rate, or exercise performance.
What it means: Garlic supplementation may improve exercise tolerance by reducing the workload on the heart. One study did not support the use of garlic in improving exercise performance in cyclists. More in human clinical studies are required to elucidate if garlic extract can improve endurance tolerance.
6. Garlic may reduce bone loss
Garlic supplements may preserve bone loss by increasing estrogen levels, reducing inflammatory cytokines, and lowering oxidative stress.
A randomized controlled trial showed that menopausal women consuming aged garlic extract had decreased inflammatory cytokine, TNF-alpha.
In ovariectomized rats, garlic oil suppressed bone resorption and prevented bone loss.
The garlic oil recovered serum estrogen, reduced oxidative stress, and protected hypogonadal bone loss in mice. 
What it means: Preliminary studies showed that garlic supplements might reduce bone loss by increasing estrogen levels and reducing inflammatory cytokines. The human studies indicating the effect of garlic on the bone loss has not yet been reported. Therefore, human studies are required to check the effectiveness of garlic supplements on reducing bone loss.
7. Garlic may treat mild to moderate lead poisoning
Preliminary studies showed that garlic supplements might improve clinical symptoms of mild to moderate lead poisoning.
Lead is a heavy metal causing toxicological effects in humans and animals.
In a study involving 117 workers at a car battery industry, garlic supplementation improved clinical signs and symptoms of lead poisoning.
The treatment group had decreased irritability, headache, and deep tendon reflex.
The consumption of garlic improved toxic effects in the lead-exposed chickens and sheep. 
What it means: Garlic supplements may reduce irritability, headache, and deep tendon reflex associated with lead poisoning. The research studies reporting the use of garlic for treating lead poisoning is scarce. Therefore, future studies will determine if garlic supplements can help in treating lead poisoning.
8. Garlic may reduce the risk of Alzheimer’s disease.
Preliminary studies showed that garlic extract might inhibit amyloidogenesis and reduce the risk of Alzheimer’s disease.
Alzheimer’s disease is characterized by the accumulation of amyloid plaques and oxidative damage.
A study reported that S-allyl-l-cysteine, a component from garlic protected neurons from amyloid proteins- induced apoptosis.
Aged garlic extract protected neuronal cells from reactive oxygen species and increased SNAP25 protein, which is essential for synaptic connections. Alzheimer’s disease has poor synaptic connections.
What it means: Some studies showed that garlic might reduce amyloid proteins induced damage by decreasing reactive oxygen species formation. Future human studies will determine if garlic can be used in reducing the risk of Alzheimer’s disease.
9. Garlic may help in the management of type II diabetes
Garlic supplements have a modest effect in reducing blood glucose levels and help in managing type II diabetes.
A meta-analysis of randomized trials showed that garlic supplements significantly reduced blood glucose levels in 1-2 weeks.
A combination of garlic extract with metformin showed a reduction in fasting blood glucose levels.
Glucose fed rats showed that raw garlic homogenate reduced plasma glucose level and improved insulin sensitivity.
What it means: Garlic extract may modestly reduce blood glucose levels and help in the management of type II diabetes. More studies are required to elucidate the effectiveness of garlic as an antidiabetic agent.
Possible Side effects of Garlic Supplements
Garlic is possibly safe when taken in recommended doses by mouth.
It can sometimes cause bad breath, burning sensation, heartburn, gas, nausea, vomiting, and diarrhea. Rarely, it may increase bleeding.
Garlic is likely safe in children, breastfeeding, and pregnant women when taken in recommended doses. Individuals with the risk of bleeding, diabetes, digestion problems, and low blood pressure should avoid taking garlic supplements.
It is recommended to stop taking garlic supplements before a scheduled surgery.
Taking garlic with isoniazid, medications for HIV/AIDS, and saquinavir might reduce the effectiveness of these drugs.
It might also interact with contraceptives, cyclosporine, medications changed by the liver, anticoagulants, and warfarin.
Dosage of Garlic Supplements
There is no recommended dose of garlic supplements.
The clinical studies used garlic supplements in the range of 600-1200mg/day, usually divided into multiple doses.
The different types of garlic supplements available are
Garlic oil: It is produced by steam distilling raw garlic. It contains original oil-soluble sulfides and no sulfur compounds.
Garlic powder: It is made by oven drying and pulverizing garlic.
Aged garlic extract: It is made by aging garlic in diluted alcohol for 20 months without heating. It has high antioxidant activity and less pungency compared to fresh garlic.
Garlic oil macerate: It is not suitable for use as a dietary supplement on a daily basis as it is high in fat.
Some animal studies showed that garlic administration might enhance memory function.
Oral administration of 250mg/kg fresh garlic homogenate improved memory function in albino Wistar rats.In diabetic rats, garlic supplementation inhibited free radical production and improved short term memory.Preliminary animal studies show some evidence that garlic supplements may enhance memory and cognition.
According to the American Institute of cancer research and the FDA, the evidence supporting the use of garlic in reducing the risk of cancer in humans is minimal.Epidemiological studies showed that garlic-based extracts decreased the risk of cancer.One study found that garlic extract supplementation lowered the mass of prostate, and reduced circulating levels of a prostate-specific antigen in prostate cancer patients. Oral ingestion of garlic reduced the risk of colon cancer in women, upper aerodigestive tract, stomach cancer, and multiple myeloma.   Although studies showed that consuming garlic may reduce the risk of cancer, randomized controlled trials studying anticancer properties of garlic are still inconclusive in humans.
Garlic supplements are gaining interest as a tick repellent. A 2001 study showed that garlic administration modestly protected against tick bites.In a study, topical garlic juice decreased northern fowl mites in hens.Garlic bulbs are rich in thiophenes and dithiane that had insect repelling properties. Further studies are required to evaluate the insect-repellent effects of garlic.
Garlic supplements are gaining attention due to the recent popularity of using natural products as alternative medicines. It has been linked to reducing high cholesterol, hypertension, and coronary artery disease. It may have a role in reducing the risk of cancer, managing diabetes, and cold infections.
Future long term and large trials are required to evaluate the effectiveness of garlic supplements in various pathological conditions and promoting general well being.