Garlic (Allium sativum) is a perennial plant and is included as a member of the Liliaceae family.  Botanically, the seeds of the garlic plant are quite unfertile and is practically considered an annual, depending on humans for propagation. Due to garlic’s culinary popularity, familiarity with the bulb of garlic is common. Garlic is divided into two groups: softneck - sativum, or cultivated garlic; and hardneck - which is of the ophioscorodon subspecies.  Contrary to popular belief, garlic does not exist in the wild. The majority of garlic bulbs consumed in human nutrition are cultivated, with the majority of product coming from Central California.  There are many cultivars of garlic, including Italian Purple and Silverskin. The bulb is the part of the plant that is used medicinally as well. Extracts of garlic are often described as tasting pungent and ‘warm.’ 
There are a number of active constituents in garlic. However, one particular category is responsible for majority of its credited medicinal activity. The sulfur-containing compounds, sulfoxides (alliin) and thiocyanates, are the primary constituents of interest. Volatile oils are are also significant, making up 0.1 - 0.3% of the bulb, and are composed of about 14 different components. Garlic also contains; vitamin A, thiamine, nicotinaminde, vitamin C, protein, high concentration of trace minerals (i.e. selenium), glucosinolates, enzymes (i.e. alliinase, peroxidase, myrosinase), choline, iodine, and saponins. [4, 5]
The sulfur containing compound, alliin, has been of particular interest to researches concerning its possible therapeutic implications on human health. Alliin is exposed to the enzyme alliinase when the bulb is crushed, resulting in the creation of allicin. Interestingly, alliin itself is odorless until it is crushed or chewed. Only upon its conversion to allicin does it obtain its pungent smelling odor.  The entire process takes approximately six seconds. Allicin is readily absorbed into the bloodstream and eliminated primarily via the lungs and skin. It is unstable agent that is immediately converted to numerous other organosulfur compounds, including ajoene, trisulfide and diallyl disulfide. 
Supplements can be purchased as dried garlic powder, keeping a consistent level of alliin and allinase; odorless, aged garlic extract (which is rich in modified sulfur components that also have medicinal activity); and steam distilled garlic oil containing diallyl sulfides.  There are also enteric coated products which prevent digestion of the capsule in the stomach where hydrochloric acid inactivates the enzyme allinase, which would likely prevent its subsequent conversion to allicin. These preparations favor the chemical activity associated with allicin.
Medicinal actions ascribed to garlic:
- antimicrobial (antibacterial and antimycotic)
- counter irritant
- digestive aid
- anti-platelet aggregant
Garlic is an herbal medicine that is used for a panacea of disease conditions and disorders. Nevertheless, much of the research on garlic has focused on specific cardiovascular and immune applications.
A double-blind randomized, placebo-controlled intervention study was conducted in 46 hypercholesterolemic subjects who had failed, or were not compliant, with drug therapy.  Enteric-coated Australian garlic powder tablets with 9.6 milligrams allicin-releasing potential or placebo was administered to test subjects over a twelve week period. The results showed that the subjects taking garlic had a significant reduction in total cholesterol and LDL-cholesterol, with significant increases in HDL cholesterol levels; while the placebo group had a non-significant increase in total cholesterol and LDL-C. Additionally, no significant difference in triglycerides or in LDL/HDL ratio was observed between groups. The authors conclude that the garlic preparation used in this study can have value in mild to moderate hypercholesterolemic patients when combined with a low fat diet. Furthermore, two meta-analyses concluded that garlic has a lipid-lowering effect of 9 - 12%. [9, 10]
In addition to its beneficial effects on cholesterol, garlic has also demonstrated anti-platelet aggregation effects which is important in thrombus formation. A 10-month study comparing the effect of aged garlic extract (AGE) with placebo on the lipid profiles of moderately hypercholesterolemic men was conducted.  Platelet functions and susceptibility of lipoproteins to oxidation in a subgroup of this study population was also examined. The results showed that subjects supplemented with 7.2 AGE per day showed a significant reduction of epinephrine- and collagen-induced platelet aggregation.
Platelet adhesion to fibrinogen was reduced by approximately 30% in the AGE group when compared with placebo. A trend toward decreased susceptibility of lipoproteins to oxidation was also observed. The conclusion of the study was that the beneficial effect of garlic preparations on lipids and blood pressure also extends to platelet function, providing a wider potential protection of the cardiovascular system.
Garlic’s antimicrobial activity makes it a prime candidate for the treatment of infectious disease. A randomized double-blind, placebo-controlled trial of 146 volunteers was conducted in the United Kingdom to identify the effect of garlic administration in the prevention of the common cold.  Over a 12-week period between November and February, subjects received an allicin-containing garlic capsule or placebo. They recorded any symptoms or incidence of colds in a diary using a five point assessment scale. Analysis of the study showed that subjects taking the garlic supplement had significantly fewer colds than the placebo group, and recovered faster when infection occurred.
Antimycotic activity has also been documented for garlic. A randomized trial of soldiers with diagnosed dermatophytoses was conducted.  Subjects were divided, randomly, into two treatment groups: one that received standard prescription topical application of 0.1% terbinafine, the other topical 0.6% ajeone - an organosulfur compound of garlic. After 60 days of treatment, the healing rate of 73% and 71% was observed in the groups treated with ajoene and terbinafine, respectively. The authors concluded that, together with previous studies, ajoene is a new agent for the topical treatment of superficial mycoses; for the first time showing the therapeutic usefulness of an inhibitor of phospholipids biosynthesis in eukaryotes.
Garlic has also been studied in the treatment of various types of cancer. In a study of 21 patients with basal cell carcinoma, a topical preparation of ajoene, an organosulfur compound of garlic, was applied to the tumor sites.  In 17 of these patients, tumor size was reduced. The mechanism of action was enabled via induction of the mitochondria-dependent route of apoptosis.
A meta-analysis was also conducted for all cancers mutually and separately for colorectal and stomach cancers; in relation to consumption of exclusively raw garlic, cooked garlic, or both.  The authors concluded that high intake of raw and cooked garlic may be associated with a protective effect against stomach and colorectal cancers.
In addition, garlic may be beneficial as an oral rinse, and potential treatment for hypertension, atherosclerosis, HIV infection, intermittent claudication, recurrent vaginal infections, diabetes mellitus, and erectile dysfunction. [16-20]
Research has been conducted on many of the preparation types discussed in the introduction. When choosing a particular product, close attention should be paid to the type of preparation (i.e. aged garlic or enteric coated garlic) used in research trials. Taking note of the dosage range used for particular conditions is equally important. Generally, 4000 milligrams (mg) of fresh garlic = 10 mg alliin = 4000 μg allicin potential.  A common dose would yield approximately 5000 μg allicin potential daily for the treatment of actual disease conditions.  For preventative or wellness purposes, half that dose is likely sufficient.
Caution must be taken in patients with blood clotting disorders, including hypercoagulation due to the possibilty of embolic complications. Garlic should not be used within 10 days of surgery. 
Garlic can destroy vitamin K producing bacteria in the gut. A dose of 3 grams daily for 26 weeks can also reduce platelet aggregation. Therefore, patients on anticoagulant therapy should have PT and INR’s monitored to stabilize the dosage of coumadin. 
Garlic may interact with certain prescription medications. The chemcial constituents of garlic can modulate the activity of drug-metabolizing enzymes (notably cytochrome p450 isozymes) and/or the drug transporter P-glycoprotein, impacting the levels of many prescription drugs, including anti-cancer agents.  For this reason, garlic use should be disclosed to providers to ensure that patients on drug therapies maintain adequate blood levels of prescribed drugs.
Garlic has also been shown to produce hypoglycemia when taken in combination with chlorpropamide. 
Garlic is generally well tolerated with few side effects. However, adverse effects may include irritations to the gastric mucosa in sensitive individuals and garlic breath; which is a common complaint that passes quickly. When garlic exits the lungs and skin, the odor is noticeable. Patients should be aware of this aspect of garlic therapy as well. Although rare, there have also been case reports citing allergic reactions.
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