How we rate
“Strong” = several concordant meta-analyses or systematic reviews. “Moderate” = one synthesis, or several controlled trials. “Limited” = early trials only. The rating is about the quality of the studies, not about how useful the supplement is.
Garlic
Lowers blood pressure slightly, especially in hypertensive individuals; on cholesterol and the common cold, studies are contradictory or lacking.
Benefits
- Lowers blood pressure: −3.75 mmHg for systolic and −3.39 mmHg for diastolic in a meta-analysis of 17 trials, with a more pronounced effect in hypertensives
- In hypertensives poorly controlled despite their treatment, aged garlic extract lowered systolic blood pressure by 10.2 mmHg (960 mg per day, 50 patients) and by 11.8 mmHg (480 mg per day, 79 patients) more than placebo in 12 weeks
- Fewer colds in one trial: 24 colds versus 65 with placebo in 146 volunteers for 12 weeks, with faster recovery
- In 120 healthy adults, 2.56 g of aged garlic extract per day for 90 days made colds and the flu less severe, with fewer days of absence, without reducing their number
- Lowers fasting blood glucose (meta-analysis of 7 trials, 513 people)
- In 80 diabetics, 2,400 mg of aged garlic extract per day caused a type of vulnerable plaque in the heart's arteries to regress, compared to placebo
Precautions
The most common complaints are breath and body odor; allergies, skin burns, and platelet alteration with a risk of hemorrhage have been reported, and 13 weeks of aged garlic extract slowed platelet aggregation. With anticoagulants, studies are divided: an adverse effect review notes that garlic may enhance the effect of warfarin, while two trials find nothing (12 healthy men; 52 patients on warfarin, with no more hemorrhages over 12 weeks of aged garlic extract). If you are taking HIV treatment, watch out: in 10 volunteers, garlic capsules lowered the amount of saquinavir in the blood by 51%. In a meta-analysis of observational studies, garlic supplement users had a slightly higher risk of colorectal cancer (relative risk of 1.18), which this type of study does not allow to be attributed to garlic. The limitations: on cholesterol, studies are contradictory — a meta-analysis of 26 studies finds a drop in total cholesterol and triglycerides, another of 13 trials (1,056 people) finds nothing, and two trials (raw garlic and supplements over 6 months, garlic oil over 12 weeks) do not either. A review of 32 trials on blood pressure rates their quality as generally low and does not recommend garlic as advice against hypertension. Against the common cold, the Cochrane review included only a single trial and judges the evidence insufficient. In a 22-year follow-up of 3,365 residents of a high-risk region, garlic did not reduce the number of stomach cancers, only mortality.
Recommended dosage
480 mg to 1.2 g of aged garlic extract per day in trials on blood pressure
How to take it
- Studies often involve only sick individuals: recent trials on pressure concern poorly controlled hypertensives, who took garlic in addition to their treatment.
- Blood pressure: 480 mg, 960 mg, or 1.2 g of aged garlic extract per day for 12 weeks, depending on the trial.
- Common cold: an allicin garlic supplement for 12 weeks in the only prevention trial; 2.56 g of aged garlic extract per day for 90 days in the symptom severity trial.
- Arterial plaque: 2,400 mg of aged garlic extract per day in diabetics, in a single trial.
- Raw garlic or supplement: at the dose of one clove per day, neither raw garlic, powder, nor aged extract lowered cholesterol in 6 months in 192 adults.
- None of the studies in this product profile compare the timing of intake or taking it with or between meals.
Did you know?
In a 12-week trial of 146 volunteers, the garlic group had 24 colds compared to 65 for the placebo — but this is the only trial the Cochrane review was able to include, and it judges the evidence insufficient.
What the studies say
This meta-analysis, based on 17 randomized controlled trials, studied the impact of garlic supplements on blood pressure. The results show a significant reduction in systolic (3.75 mmHg) and diastolic (3.39 mmHg) blood pressure with garlic intake compared to control groups. This blood-pressure-lowering effect is particularly notable in hypertensive patients.
Wang HP, Yang J, Qin LQ, Yang XJ. — Journal of clinical hypertension (Greenwich, Conn.) (2015) — View the source study
For information only: not a substitute for medical advice. Dietary supplements are not a replacement for a varied, balanced diet. Learn more