Garlic - Drug Monograph

Introduction

Garlic (Allium sativum) is a widely used herbal supplement with a long history of medicinal use dating back thousands of years. While not classified as a pharmaceutical drug, garlic preparations are commonly used for their potential cardiovascular benefits, antimicrobial properties, and other health effects. This monograph provides evidence-based information on garlic's pharmacological properties and clinical applications.

Mechanism of Action

Garlic's pharmacological effects are primarily attributed to its sulfur-containing compounds, particularly allicin, which is formed when garlic is crushed or chopped. Key mechanisms include:

• Inhibition of HMG-CoA reductase and cholesterol synthesis
• Antiplatelet activity through inhibition of thromboxane synthesis
• Vasodilation via increased nitric oxide production
• Antimicrobial activity through thiosulfinate compounds
• Antioxidant effects through scavenging reactive oxygen species
• Potential modulation of cytochrome P450 enzymes

Indications

While not FDA-approved for specific medical indications, garlic is commonly used for:

• Mild hypertension management (systolic reduction of ~7-16 mmHg)
• Mild hypercholesterolemia (LDL reduction of ~10-15%)
• Cardiovascular risk reduction
• Antimicrobial applications (limited evidence)
• Antioxidant support

Dosage and Administration

• Fresh garlic: 2-5 g daily (approximately 1-2 cloves)
• Dried powder: 600-900 mg daily
• Aged garlic extract: 600-1200 mg daily
• Garlic oil: 5 mg daily

• Typically administered orally with meals
• Dosing frequency: Usually once or twice daily
• Duration: Effects may require 4-12 weeks of consistent use

• Elderly: No specific dosage adjustments recommended
• Hepatic impairment: Use with caution
• Renal impairment: No specific recommendations

Pharmacokinetics

Contraindications

• Known hypersensitivity to garlic or related plants (onions, leeks)
• Active bleeding disorders or thrombocytopenia
• Pre-operative period (typically 7-10 days before surgery)
• Concomitant use with strong anticoagulants without medical supervision

Warnings and Precautions

• Bleeding risk: May potentiate anticoagulant effects
• GI effects: May cause heartburn, flatulence, and GI discomfort
• Surgical considerations: Discontinue prior to elective surgery
• Pregnancy: Limited safety data; use only if potential benefit justifies risk
• Lactation: May alter milk flavor; safety not established
• Diabetes: May enhance hypoglycemic effects

Drug Interactions

• Anticoagulants: Warfarin, heparin, direct oral anticoagulants (increased bleeding risk)
• Antiplatelet agents: Aspirin, clopidogrel (additive effects)
• Protease inhibitors: May decrease saquinavir levels
• Hypoglycemic agents: May enhance insulin and oral hypoglycemic effects
• Cytochrome P450 substrates: Potential modulation of drug metabolism

Adverse Effects

• Breath and body odor
• Heartburn
• Flatulence
• Nausea
• Diarrhea

• Allergic reactions (rare)
• Increased bleeding tendency
• Chemical burns (with topical application)
• Asthma exacerbation (in sensitive individuals)

Monitoring Parameters

• Lipid profile (baseline and every 3-6 months)
• Blood pressure (regular monitoring)
• Bleeding parameters (INR if on anticoagulants)
• Liver function tests (periodically with long-term use)
• Signs of bleeding or bruising
• Glucose levels in diabetic patients

Patient Education

• Inform healthcare providers about garlic supplement use
• Discontinue use at least 2 weeks before surgical procedures
• Be aware of potential bleeding risks and report unusual bruising
• Monitor for gastrointestinal discomfort
• Understand that effects may take several weeks to manifest
• Choose standardized preparations for consistent dosing
• Store supplements properly to maintain potency

References

1. Ried K. Garlic lowers blood pressure in hypertensive subjects, improves arterial stiffness and gut microbiota: A review and meta-analysis. Exp Ther Med. 2020;19(2):1472-1478. 2. Wang HP, Yang J, Qin LQ, Yang XJ. Effect of garlic on blood pressure: a meta-analysis. J Clin Hypertens. 2015;17(3):223-231. 3. Sun YE, Wang W, Qin J. Anti-hyperlipidemia of garlic by reducing the level of total cholesterol and low-density lipoprotein: A meta-analysis. Medicine. 2018;97(18):e0255. 4. Borrelli F, Capasso R, Izzo AA. Garlic (Allium sativum L.): adverse effects and drug interactions in humans. Mol Nutr Food Res. 2007;51(11):1386-1397. 5. National Center for Complementary and Integrative Health. Garlic. NCCIH Publication No. D425. 2020. 6. European Medicines Agency. Assessment report on Allium sativum L., bulbus. EMA/HMPC/7686/2013. 2016.