Benefits
Blood pressure reduction
Across 108 randomized trials in 7,137 adults, garlic supplements lowered systolic blood pressure by 3.7 mmHg and diastolic by 2.0 mmHg more than control. In nine trials of 482 adults with high blood pressure the systolic fall was about 9 mmHg, but the trials disagreed sharply and the effect shrank in the better-conducted ones. A review of placebo-controlled trials found none that reported heart attacks, strokes or deaths.
Cholesterol and cardiovascular risk markers
Pooled trials find garlic lowers total cholesterol by roughly 10 to 17 mg/dL and LDL by roughly 6 to 9 mg/dL, mainly in adults whose cholesterol was already raised, with a small rise in HDL. Results depend on the trial: a six-month study of raw garlic, garlic powder and aged garlic extract at about a clove a day found no LDL change with any form. A small one-year trial of aged garlic extract reduced one of four coronary plaque measures.
Immune defense during the winter months
In one 146-person trial, a daily allicin garlic supplement over winter was followed by 24 self-reported colds against 65 on placebo; a Cochrane review found this single trial was the only one meeting its standard and called the evidence insufficient. A 120-person trial of aged garlic extract found no significant difference in how many colds or flu episodes people caught, though the garlic group reported fewer symptoms.
Fasting blood sugar and HbA1c
Across 27 randomized trials in 1,619 adults, garlic lowered fasting blood glucose by about 2.8 mg/dL and HbA1c by about 0.5 percentage points, with no significant change in fasting insulin or HOMA-IR, although a 108-trial analysis did find small falls in both. In a 24-week trial in 60 adults on metformin, adding garlic powder lowered fasting blood sugar by about 3 percent against 0.6 percent with placebo plus metformin.
Mechanism of action
Lipid synthesis and platelet aggregation
In laboratory studies, garlic inhibits enzymes involved in cholesterol and fat synthesis and reduces the ability of platelets to clump together. A critical review of trials found that platelet aggregation was the most marked effect in people, while results for blood pressure and oxidative stress were less clear and only 44% of trials since 1993 showed a fall in total cholesterol.
ACE inhibition and antioxidant activity
Cell and laboratory work shows garlic inhibiting angiotensin-converting enzyme and protecting LDL and red blood cells from oxidation. The same review notes that differing garlic preparations, unclear active compounds, short trials and poor randomization limit how far these findings carry over to people.
Clinical trials
Double-blind randomized placebo-controlled trial of Kyolic aged garlic extract, 1.2 g/day providing 1.2 mg S-allylcysteine, for 12 weeks; 72% of participants were on blood pressure medication and 25% on blood-thinning medication. Supported by a grant from Wakunaga of America, which makes the extract, supplied the capsules and funded tests and research assistance; the paper states the company had no role in design, analysis or writing. (Ried et al. 2016, Integr Blood Press Control)
88 general practice patients and community members with uncontrolled hypertension. 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 chosen because it responded is not a fair estimate of the average effect. Dropout was 22% on placebo and 9% on garlic. Central pressure, arterial stiffness and cholesterol only trended in garlic's favor.
Prospective randomized double-blind trial of aged garlic extract 2,400 mg/day versus placebo in adults with metabolic syndrome, with cardiac CT angiography at baseline and about a year later. Funded by Wakunaga of America, which makes the extract. ClinicalTrials.gov NCT01534910. (Matsumoto et al. 2016, J Nutr)
55 adults, mean age 59, 71% men; 27 on the extract and 28 on placebo. 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=0.0049). 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. No heart attacks, strokes or deaths were counted, and the authors call for further studies on whether the change matters.
Parallel four-arm randomized trial of raw garlic, a powdered garlic supplement, an aged garlic extract supplement or placebo, each at about one 4 g clove equivalent, 6 days a week for 6 months, in adults with LDL cholesterol of 130 to 190 mg/dL; primary outcome LDL cholesterol; funded by the US National Institutes of Health. (Gardner et al. 2007, Arch Intern Med)
192 adults; 87% to 90% retention in all four arms.
None of the three garlic forms changed the primary outcome compared with placebo. Six-month mean changes were +0.4 mg/dL with raw garlic, +3.2 with the powder and +0.2 with aged extract, against -3.9 on placebo. No secondary blood fat measure differed between groups either.
Double-blind randomized trial of one allicin-containing garlic capsule or placebo daily for 12 weeks from November to February, with self-reported diary outcomes; the sole author was based at the Garlic Center, East Sussex. A Cochrane review rated this the only trial meeting its standard but judged a single trial insufficient, and reported recovery time as similar between groups (4.63 versus 5.63 days). (Josling 2001, Adv Ther)
146 adult volunteers.
The garlic group recorded 24 common cold episodes against 65 on placebo (P<.001) and 111 days of illness against 366. The paper reported a shorter symptom duration with garlic, 1.52 versus 5.01 days. All outcomes were self-reported in diaries.
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; the primary aim was immune cell proliferation, and illness incidence was a secondary outcome. Registered as NCT01390116, which lists Wakunaga Pharmaceutical, the extract's maker, as an industry collaborator. (Nantz et al. 2012, Clin Nutr)
120 healthy adults, 60 per group; cell measurements in a 56-person subset.
The number of colds and flu episodes did not differ significantly 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. Gamma-delta T cell and NK cell proliferation were greater than placebo at 45 days. Cells proliferating in a laboratory assay are not the same as getting sick less often.
Randomized trial of garlic powder tablets (KWAI) 300 mg three times daily plus metformin 500 mg twice daily versus placebo plus metformin for 24 weeks in adults with type 2 diabetes and fasting blood sugar above 126 mg/dl. The garlic group also showed small falls in total cholesterol (6.2 mg/dl), LDL (3 mg/dl) and triglycerides and a rise in HDL versus placebo. Funding is not stated in the abstract. (Ashraf et al. 2011, Pak J Pharm Sci)
60 adults on metformin, 30 per group.
Fasting blood sugar fell 3.12% with garlic plus metformin at week 24 against a 0.59% change with placebo plus metformin; the abstract calls this a significant reduction (P<0.005) compared with the placebo group. It reports only percentage changes for this outcome and does not make clear whether the P value is the between-group test. The trial was small.