Garlic

Metadata

Approved Indications

Commission E (Germany)

• Approved: Supportive to dietary measures at elevated levels of lipids in blood
• Approved: Preventive measures for age-dependent vascular changes (atherosclerosis prophylaxis)
• No known contraindications listed by Commission E

ESCOP Monograph

• Therapeutic indications:
  • Prophylaxis of atherosclerosis
  • Treatment of elevated blood lipid levels insufficiently controlled by diet
  • Supportive treatment of hypertension
  • Reducing severity of upper respiratory tract infections and catarrhal conditions

WHO Monograph (1999)

• Uses supported by clinical data:
  • Adjuvant to dietetic management in treatment of hyperlipidemia
  • Prevention of atherosclerosis
  • Age-dependent vascular changes
  • Mild hypertension

EMA/HMPC

• Traditional use registration for various garlic preparations
• Lower regulatory tier than “well-established use” for cardiovascular claims

Agreement/Disagreement Between Bodies

• Agreement on atherosclerosis prophylaxis and lipid-lowering support across Commission E, ESCOP, and WHO
• ESCOP goes further than Commission E by explicitly including hypertension
• EMA/HMPC is more conservative, granting only traditional use status for most preparations
• The clinical evidence has weakened somewhat since the Commission E and ESCOP monographs were written, as newer, better-designed trials show more modest lipid effects

Conditions Treated

Primary (evidence-based)

• Hyperlipidemia (adjunctive to dietary measures) — modest effect
• Atherosclerosis prophylaxis (plaque progression inhibition) — more promising
• Mild hypertension (supportive)

Secondary/Traditional

• Peripheral vascular disease
• Platelet aggregation inhibition
• General cardiovascular risk reduction
• Upper respiratory infections (non-cardiovascular indication)

Mechanism of Action

Lipid-Lowering

• Inhibition of HMG-CoA reductase (similar mechanism to statins, but much weaker)
• Inhibition of cholesterol and fatty acid synthesis in hepatocytes
• Enhanced bile acid excretion
• Active compounds: allicin, S-allyl cysteine, ajoene

Anti-Atherogenic

• Inhibition of LDL oxidation (antioxidant activity)
• Reduction of smooth muscle cell proliferation in vessel walls
• Anti-inflammatory effects on vascular endothelium
• Direct inhibition of atherosclerotic plaque formation

Antihypertensive

• Stimulation of nitric oxide (NO) production
• Hydrogen sulfide (H2S) production via polysulfides
• Vasodilation through smooth muscle relaxation
• ACE-inhibitory activity (weak)

Antiplatelet/Anticoagulant

• Inhibition of platelet aggregation via multiple pathways
• Inhibition of thromboxane A2 synthesis
• Enhancement of fibrinolytic activity
• These effects are clinically relevant for drug interaction considerations

Key Active Constituents

• Allicin: Formed from alliin by alliinase when garlic is crushed; unstable, rapidly degraded
• Ajoene: Derived from allicin; antiplatelet and antithrombotic
• S-allyl cysteine (SAC): Stable compound in aged garlic extract (AGE); antioxidant
• Diallyl sulfides: CYP450 enzyme inducers/inhibitors
• Note: Different garlic preparations contain very different profiles of these compounds, which complicates cross-study comparisons

Clinical Evidence Summary

Meta-Analyses — Lipid Effects

Silagy & Neil 1994 (JRSM)

• 16 RCTs; garlic powder 600-900 mg/day
• Mean reduction in total cholesterol: -0.77 mmol/L (approximately 12%)
• Effect evident after 1 month, persisted for 6+ months
• No significant dose-response across 600-900 mg range [Source: pubmed.ncbi.nlm.nih.gov/8169881/]

Reinhart et al. 2009 (Ann Pharmacother)

• Updated meta-analysis
• Garlic supplementation reduced total cholesterol by -0.19 mmol/L (95% CI: -0.33 to -0.06)
• More modest effect than earlier meta-analyses
• Higher quality trials showed smaller effects [Source: pubmed.ncbi.nlm.nih.gov/19250134/]

Ried et al. 2013 (Nutr Rev)

• Garlic powder intake and cardiovascular risk factors
• Consistent evidence for reductions in total cholesterol, LDL-cholesterol, fasting blood glucose, and blood pressure
• Effect sizes smaller than in older analyses [Source: pubmed.ncbi.nlm.nih.gov/25489404/]

Anti-Atherogenic Plaque Study

Koscielny et al. 1999 (Atherosclerosis)

• Randomized, double-blind, placebo-controlled, 4-year study
• n=152 subjects
• Plaque volumes in carotid and femoral arteries measured by B-mode ultrasound
• Continuous intake of high-dose garlic powder significantly reduced increase in atherosclerotic plaque volume
• 5-18% reduction in plaque volume in garlic group; 15.6% increase in placebo group
• This is arguably the most clinically significant cardiovascular finding for garlic [Source: pubmed.ncbi.nlm.nih.gov/10381297/]

Blood Pressure Effects

• Meta-analyses suggest modest reduction: approximately 3-5 mmHg systolic
• Larger effects in hypertensive subjects than normotensive
• Aged garlic extract (AGE) may be more effective for BP than garlic powder

European vs US/Anglophone Consensus

Safety Profile

Contraindications

• Commission E lists no contraindications
• Caution in patients with bleeding disorders
• Caution pre-operatively (discontinue 7-10 days before surgery due to antiplatelet effects)

Drug Interactions

• Anticoagulants (warfarin, heparin): May enhance anticoagulant effect. Aged garlic extract (AGE) appears relatively safe with warfarin in monitored patients [Source: pubmed.ncbi.nlm.nih.gov/16484565/]
• CYP450 interactions: Garlic extracts inhibit CYP3A4, CYP2C9, and CYP2C19. Clinically relevant interactions reported with:
  • Saquinavir (reduced plasma levels — clinically significant)
  • Warfarin (CYP2C9 substrate)
  • Other CYP3A4 substrates
• P-glycoprotein: Garlic may decrease concentrations of P-gp substrates including digoxin, colchicine, tacrolimus, and verapamil
• Hypoglycemic agents: May potentiate blood glucose lowering
• Antihypertensives: Additive hypotensive effect

Side Effects

• GI discomfort (most common): nausea, bloating, flatulence
• Garlic breath and body odor (most frequent complaint)
• Allergic reactions: contact dermatitis, urticaria, angioedema (rare), anaphylaxis (very rare)
• Burns from topical application of raw garlic (especially under occlusive dressings)
• Rare: alteration of platelet function

Pregnancy and Lactation

• Generally considered safe at food-level doses
• Supplemental doses: insufficient data; not recommended as medicine during pregnancy/lactation
• May alter breast milk flavor

Clinical Dosage

Key point: Garlic preparations are NOT equivalent. Dried garlic powder (standardized for allicin yield), aged garlic extract, garlic oil, and raw garlic contain different active compound profiles and have different evidence bases.

Sources

• German Commission E Monograph: Allii sativi bulbus
• ESCOP Monograph: Allii sativi bulbus. escop.com
• WHO Monograph: Bulbus Allii Sativi (1999)
• Silagy C, Neil A. Garlic as a lipid lowering agent — a meta-analysis. JRSM. 1994;87(1):13-16. [PubMed: 8169881]
• Reinhart KM et al. Garlic supplementation and serum cholesterol: a meta-analysis. Ann Pharmacother. 2009;43(12):1983-1988. [PubMed: 19250134]
• Koscielny J et al. The antiatherosclerotic effect of Allium sativum. Atherosclerosis. 1999;144(1):237-249. [PubMed: 10381297]
• Borrelli F et al. Garlic (Allium sativum L.): adverse effects and drug interactions in humans. Mol Nutr Food Res. 2007;51(11):1386-1397. [PubMed: 17918162]
• Macan H et al. Aged Garlic Extract May Be Safe for Patients on Warfarin Therapy. J Nutr. 2006;136(3):793S-795S. [PubMed: 16484565]

Connections

• Compare with Olive Leaf for another herb with antihypertensive properties
• See Ginkgo for another herb with antiplatelet concerns and drug interactions