Benefits
Blood pressure reduction
In the largest independent meta-analysis, 108 randomized trials in 7,137 adults, garlic supplements lowered systolic blood pressure by an average of 3.7 mmHg and diastolic by 2.0 mmHg. Meta-analyses restricted to people who already have hypertension report bigger numbers, around 8 to 9 mmHg systolic, but heterogeneity between trials is high and the effect shrank when one review restricted itself to the better-conducted trials. A 2012 Cochrane review looking for garlic trials in hypertensive patients found only two, in 87 people between them, and neither reported whether garlic changed rates of heart attack, stroke or death.
Cholesterol and cardiovascular risk markers
Meta-analyses of randomized trials find garlic lowers total cholesterol by roughly 10 to 17 mg/dL and LDL by roughly 6 to 9 mg/dL in people whose cholesterol is already raised, with a small rise in HDL and inconsistent effects on triglycerides. Imaging trials are much smaller, and most of them gave aged garlic extract combined with B vitamins, folic acid, L-arginine or coenzyme Q10, so their coronary calcium results cannot be credited to garlic on its own. The one trial of aged garlic extract alone, in 55 people with metabolic syndrome, reduced low-attenuation coronary plaque over a year but found no difference in calcified plaque or in total plaque volume.
Antimicrobial activity in the laboratory, and limited cold and flu data
Allicin kills a broad range of bacteria, fungi including Candida, and some viruses in laboratory culture. Whether swallowing garlic reproduces that inside the body is a separate question, and the human data are thin. A 2014 Cochrane review found only one trial that met its standard: 146 people took an allicin supplement or placebo for 12 weeks, and the garlic group recorded 24 colds against 65 on placebo, with fewer days ill, though recovery time once a cold started was similar. Cochrane called that single trial insufficient and said claims of effectiveness rest largely on poor-quality evidence. A separate 120-person trial of aged garlic extract found no significant difference in how many colds and flu people caught, although the garlic group logged fewer symptoms and fewer days functioning poorly, and a 56-person subset showed greater NK and gamma-delta T cell proliferation.
Blood sugar regulation
Pooled across 27 randomized trials in 1,619 adults, garlic supplements lowered fasting blood glucose by about 2.8 mg/dL and HbA1c by about 0.53 percentage points. That analysis found no significant change in fasting insulin or HOMA-IR, the usual index of insulin resistance, while a larger 108-trial analysis did report small significant falls in both, so the insulin resistance question is unsettled. Most of these trials were in people with type 2 diabetes or metabolic syndrome, and the reductions are small next to glucose-lowering medicines. The named mechanisms, alpha-glucosidase inhibition, AMPK activation and beta-cell protection, come from cell and animal work rather than from these trials.
Mechanism of action
Allicin and organosulfur compound formation
When garlic is crushed or chopped, the enzyme alliinase converts alliin to allicin (diallyl thiosulfinate), a volatile sulfur-rich compound. Allicin rapidly degrades to secondary organosulfur compounds (diallyl disulfide, diallyl trisulfide, ajoene). These are the compounds measurable in blood after garlic is eaten, and they are the presumed carriers of its cardiovascular and antimicrobial activity.
eNOS activation and nitric oxide production
Garlic organosulfur compounds activate endothelial nitric oxide synthase (eNOS), increasing bioavailable nitric oxide and promoting vasodilation, reducing blood pressure, and improving endothelial function. This mechanism is drawn largely from cell and animal work, and it is not how ACE inhibitors work.
HMG-CoA reductase and platelet aggregation inhibition
In cell studies, S-allylcysteine inhibits HMG-CoA reductase, the enzyme statins block, reducing cholesterol synthesis. Ajoene and allicin inhibit platelet aggregation by reducing thromboxane A2 synthesis and interfering with fibrinogen binding to glycoprotein IIb/IIIa receptors. These effects come from cell and ex vivo work; no trial has shown that they translate into fewer clots or fewer cardiovascular events in people.
Clinical trials
Ried K, Travica N, Sali A. Double-blind randomized placebo-controlled trial of Kyolic aged garlic extract, 1.2 g/day providing 1.2 mg S-allylcysteine, versus placebo for 12 weeks. Integr Blood Press Control 2016;9:9-21. Funded by a grant from Wakunaga of America, which makes the extract and supplied the capsules.
88 adults with uncontrolled hypertension, most already taking antihypertensive medication. 12 weeks.
Mean systolic blood pressure fell 5.0 mmHg more than with placebo (P=0.016); the overall diastolic difference was not significant. A post-hoc 'responder' subgroup showed larger falls, 11.5 mmHg systolic and 6.3 mmHg diastolic, but a subgroup picked out because it responded is not a reliable estimate of what an average person gets. Central pressure, arterial stiffness and cholesterol only trended in garlic's favour. Aged garlic extract is chemically different from raw crushed garlic, garlic powder and garlic oil, and this result does not transfer to them.
Matsumoto S, Nakanishi R, Li D, Alani A, Rezaeian P, Prabhu S, Abraham J, Fahmy MA, Dailing C, Flores F, Hamal S, Broersen A, Kitslaar PH, Budoff MJ. Prospective randomized double-blind trial of aged garlic extract 2,400 mg/day versus placebo, with cardiac CT angiography at baseline and again about a year later. J Nutr 2016;146(2):427S-432S. Funded by Wakunaga of America, which makes the extract. ClinicalTrials.gov NCT01534910.
55 adults with metabolic syndrome, mean age 59, 71% men. Scans about 354 days apart.
Low-attenuation plaque, one of four plaque measures, fell 1.5% with aged garlic extract while rising 0.2% on placebo (P=0.005). The other three measures showed no difference between groups: total plaque volume, non-calcified plaque, and calcified plaque, which changed by 0.2% in both arms. This was a small single-centre imaging study; no heart attacks, strokes or deaths were counted, and the authors themselves call for further work on whether the plaque change matters clinically.
Nantz MP, Rowe CA, Muller CE, Creasy RA, Stanilka JM, Percival SS. Randomized, double-blind, placebo-controlled trial of encapsulated aged garlic extract 2.56 g/day versus placebo over 90 days of the cold and flu season. Clin Nutr 2012;31(3):337-44.
120 healthy adults, 60 per group; the immune cell measurements were made in a 56-person subset.
The number of colds and flu episodes was not significantly different from placebo. Among the illnesses that did occur, the garlic group logged 21% fewer symptoms, 61% fewer days functioning sub-optimally and 58% fewer missed work or school days. In a 56-person subset, gamma-delta T cell and NK cell proliferation were greater than placebo at 45 days. Industry-funded. Cells proliferating in a laboratory assay are not the same thing as getting sick less often, and on that outcome this trial was null.