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Table of Contents > Interactions & Depletions > Garlic (Allium sativum L.) Print

Garlic (Allium sativum L.)



Interactions

Garlic/Drug Interactions:
  • AcetaminophenAcetaminophen: Garlic ingestion was associated with a slight increase in sulfate conjugation of acetaminophen in humans (264; 265).
  • AnthelminticsAnthelmintics: Based on laboratory study, garlic hexane extract may be effective against the Caillaria species (266). The exact mechanism is not well understood.
  • AntibioticsAntibiotics: In vitro, garlic exerted activity against multiple pathogenic bacteria (267; 268; 150; 269; 270; 167; 271; 272; 273), including Streptococci (274), Mycobacteria (275; 276; 277; 278), and Helicobacter pylori (279; 280; 167; 281). Theoretically, concurrent use of garlic and antibacterial agents may have additive effects.
  • Anticoagulants and antiplateletsAnticoagulants and antiplatelets: In theory, the risk of bleeding may be increased by concomitant use of garlic and agents with anticoagulant or antiplatelet effects. Bleeding has been associated with garlic use in several studies and case reports, including intraoperatively, possibly related to impaired platelet aggregation or increased fibrinolysis (93; 94; 95; 96; 97; 98; 99; 95; 100; 101; 35; 102; 103; 104; 105; 106). In contrast, there are several reports of suboptimal quality that suggest a lack of an effect on ex vivo platelet aggregation (176) or bleeding or hemorrhaging in patients on warfarin anticoagulation therapy (256). Additionally, in human study, coadministration of warfarin and garlic did not alter the pharmacokinetics and pharmacodynamics of warfarin (256; 282).
  • Antidiabetic agentsAntidiabetic agents: The available evidence suggests that garlic does not lower blood glucose levels in humans. Although animal studies have reported that garlic or its constituents (such as S-allyl cysteine sulfoxide [SACS]) may decrease glucose concentrations and increase insulin secretion (107; 108), multiple human trials have failed to demonstrate significant effects of oral garlic preparations on measures of glycemic control in diabetic or nondiabetic patients (113; 171; 105; 122; 123; 124; 246; 128; 130; 245). One study of suboptimal quality noted a small, significant reduction in mean blood glucose levels in patients treated with 800mg of daily dehydrated garlic (Kwai®), from 89mg/dL to 79mg/dL over four weeks (127). Although the available studies have been small (<100 subjects) with methodological weaknesses, it appears that garlic likely does not exert clinically relevant effects on glucose levels. Evaluations of possible interactions with hypoglycemic agents are lacking.
  • AntifungalsAntifungals: In laboratory study, ajoene derived from garlic displayed antifungal activity; however, the mechanism of action is not well understood (283). Based on laboratory study, allicin enhanced the fungicidal activity of amphotericin B (synergistic effects) (284; 285; 286). Theoretically, concurrent use may modulate the effectiveness of antifungals.
  • Antiglaucoma agentsAntiglaucoma agents: Based on animal study, S-allylmercaptocysteine, a garlic-derived compound, reduced intraocular pressure and may involve the elevation of ANP levels (49). Theoretically, concurrent use may modulate the effectiveness of antiglaucoma agents.
  • AntihypertensivesAntihypertensives: Numerous studies have reported small mean reductions in systolic and diastolic blood pressure associated with the use of oral garlic vs. placebo, and combination use with antihypertensives may result in additive effects (2; 112; 113; 114; 115; 116; 105; 117; 118; 119; 120; 121; 122; 123; 124; 124; 125; 126; 127; 102; 128; 129; 130; 131; 132; 133; 134; 135; 7; 136; 137; 3; 138; 139; 140; 141; 142).
  • Antilipemic agentsAntilipemic agents: Multiple trials have demonstrated modest lipid-lowering properties of oral garlic supplementation, including decreases in total cholesterol and low-density lipoprotein (2; 3; 5; 6; 4). Effects may be additive with other lipid-lowering agents.
  • Antineoplastic agentsAntineoplastic agents: In clinical trials, aged garlic extract supplementation has resulted in reduced progression of colorectal adenomas (179; 180). Thus, garlic and antineoplastic agents may have additive effects.
  • Antiobesity agentsAntiobesity agents: Based on animal study, the allyl-containing polysulfides in garlic may increase thermogenesis (43). Theoretically, concurrent use may modulate the effectiveness of antiobesity agents.
  • Antiretroviral agentsAntiretroviral agents: Garlic supplementation was shown to cause a significant decrease in plasma concentrations of saquinavir taken at a dose of 1,200mg three times daily by 10 healthy volunteers (109; 110; 111). Combination use may result in diminished effects of saquinavir. However, a preliminary study in 10 healthy adults found no significant effects on ritonavir levels following eight doses of 10mg of Natural Source Odourless Garlic® (287). Theoretically, concurrent use may modulate the effectiveness of antiretroviral agents.
  • Cytochrome P450-metabolized agentsCytochrome P450-metabolized agents: In human study, garlic oil reduced cytochrome P450 2E1 activity (143; 144). In human study, allicin reduced the metabolism of omeprazole by inhibition of CYP2C19 in individuals with the CYP2C19*1/CYP2C19*1 and CYP2C19*1/CYP2C19*2 or *3 genotypes, but not in those with the CYP2C19*2/ CYP2C19*2 genotype (145). Allicin has been found to reduce the effectiveness of cyclosporine by increasing CYP3A4 activity (146). Although animal and human studies suggest possible induction or inhibition of various P450 enzymes (145; 143; 147; 148), other study has found garlic to have a lack of an effect on the metabolism of CYP450 substrates (alprazolam, P450 3A4 or dextromethorphan, P450 2D6) (149).
  • DisulfiramDisulfiram: In theory the high levels of alcohol found in garlic tinctures may cause nausea or vomiting when taken with disulfiram (Antabuse®).
  • Drugs used for osteoporosisDrugs used for osteoporosis: Based on animal study, garlic may suppress bone loss owing to estrogen deficiency (69; 70). Theoretically, concurrent use may modulate the effectiveness of drugs used for osteoporosis.
  • EstrogensEstrogens: Based on animal study, garlic may have phytoestrogenic activity (71). Theoretically, concurrent use with estrogens may have additive effects.
  • Fertility agentsFertility agents: Based on animal evidence, chronic garlic ingestion for 70 days may be associated with suppression of spermatogenesis (251). Theoretically, concurrent use may modulate the effectiveness of fertility agents.
  • Hematological agentsHematological agents: In theory, the risk of bleeding may be increased by concomitant use of garlic and hematologic agents. Bleeding has been associated with garlic use in several studies and case reports, including intraoperatively, possibly related to impaired platelet aggregation or increased fibrinolysis (93; 94; 95; 96; 97; 98; 99; 95; 100; 101; 35; 102; 103; 104; 105; 106). In contrast, there are several low-quality reports that suggest no effect on ex vivo platelet aggregation (176) or bleeding or hemorrhaging in patients on warfarin anticoagulation therapy (256).
  • ImmunosuppressantsImmunosuppressants: Based on laboratory and animal study, garlic extract may enhance immune function (288; 289; 290; 291; 292; 293; 294; 288; 289; 290; 291; 292; 293). Theoretically, concurrent use may modulate the effectiveness of immunosuppressants.
  • IsoniazidIsoniazid: In animal study, crude aqueous extract of garlic reduced the maximum concentration and area under the curve (AUC) of garlic; a lack of an effect was noted on half-life (295).
  • MetronidazoleMetronidazole: In theory the high levels of alcohol found in garlic tinctures may cause nausea or vomiting when taken with metronidazole (Flagyl®).
  • Neurologic agentsNeurologic agents: In animal models, aged garlic extract prevented deterioration of hippocampal-based memory tasks (296) and had antiamyloidogenic effects (14). In vitro, S-allyl-L-cysteine protected against amyloid beta- and tunicamycin-induced neuronal death (297). The protein TRPA1 mediates the response to pungent irritants found in garlic and is found mainly in nociceptive neurons of peripheral ganglia and in all the mechanosensory epithelia of the inner ear (298).
  • Nonsteroidal anti-inflammatory agentsNonsteroidal anti-inflammatory agents: Bleeding has been associated with oral garlic use in several studies and case reports, including intraoperatively, possibly related to impaired platelet aggregation or increased fibrinolysis (93; 150; 96; 97; 98; 99; 95; 100; 101; 35; 102; 103; 104; 105). In theory, combined use of garlic and NSAIDS may increase the risk of bleeding.
  • Performance-enhancing agentsPerformance-enhancing agents: A single administration of garlic resulted in increased endurance performance, likely due to increase in fibrinolytic activity in the resting state (178). Theoretically, concurrent use may have additive effects.
  • Potassium saltsPotassium salts: Garlic is a source of potassium (299) and may elevate potassium levels. Theoretically, concurrent use of garlic and potassium supplements may increase the risk of hyperkalemia.
  • Thyroid hormones iodineThyroid hormones, iodine: Hypothyroidism and reduced iodine uptake by the thyroid have been reported anecdotally with garlic use, which may alter the effects of thyroid agents.
  • VasodilatorsVasodilators: In animal and human study, garlic and its major constituent, allicin, had significant vasodilator activity via hyperpolarization of vascular smooth muscle (151; 152; 153; 154). In animal study, diallyldisulfide and allyl mercaptan, metabolites of allicin, did notpossess this vasodilatory action (154). Theoretically, concurrent use of garlic and other vasodilators may have additive effects.

Garlic/Herb/Supplement Interactions:
  • AnthelminticsAnthelmintics: Based on laboratory study, garlic hexane extract may be effective against the Caillaria species (266). The exact mechanism is not well understood.
  • AntibacterialsAntibacterials: In vitro, garlic exerted activity against multiple pathogenic bacteria (267; 268; 150; 269; 270; 167; 271; 272; 273), including Streptococci (274), Mycobacteria (275; 276; 277; 278) and Helicobacter pylori (279; 280; 167; 281). Theoretically, concurrent use of garlic and antibacterial agents may have additive effects.
  • Anticoagulants and antiplateletsAnticoagulants and antiplatelets: In theory, the risk of bleeding may be increased by concomitant use of garlic and agents with anticoagulant or antiplatelet effects. Bleeding has been associated with garlic use in several studies and case reports, including intraoperatively, possibly related to impaired platelet aggregation or increased fibrinolysis (93; 94; 95; 96; 97; 98; 99; 95; 100; 101; 35; 102; 103; 104; 105; 106). In contrast, there are several reports of suboptimal quality that suggest a lack of an effect on ex vivo platelet aggregation (176) or bleeding or hemorrhaging in patients on warfarin anticoagulation therapy (256). Additionally, in human study, coadministration of warfarin and garlic did not alter the pharmacokinetics and pharmacodynamics of warfarin (256; 282).
  • AntifungalsAntifungals: In laboratory study, ajoene derived from garlic displayed antifungal activity; however, the mechanism of action is not well understood (283). Based on laboratory study, allicin enhances the fungicidal activity of amphotericin B (synergistic effects) (284; 285; 286). Theoretically, concurrent use may modulate the effectiveness of antifungals.
  • AntilipemicsAntilipemics: Multiple trials have demonstrated modest lipid-lowering properties of oral garlic supplementation, including decreases in total cholesterol and low-density lipoprotein (2; 3; 5; 6; 4). Theoretically, effects may be additive with other lipid-lowering agents.
  • AntineoplasticsAntineoplastics: In clinical trials, aged garlic extract supplementation has resulted in reduced progression of colorectal adenomas (179; 180). Theoretically, garlic and antineoplastics may have additive effects.
  • Antiobesity herbs and supplementsAntiobesity herbs and supplements: Based on animal study, the allyl-containing polysulfides in garlic may increase thermogenesis (43). Theoretically, concurrent use may modulate the effectiveness of antiobesity herbs and supplements.
  • AntioxidantsAntioxidants: In various studies, garlic and its constituents displayed antioxidant activity: increasing activities of antioxidant enzymes (glutathione peroxidase, catalase, and superoxide dismutase), lowering xanthine oxidase activity (i.e., eliminating oxidant stress), and inhibiting lipid peroxidation and prostaglandin production (300; 301; 302; 303; 304; 305; 306; 30; 40; 301; 307; 53; 308; 309; 310; 311; 312; 313; 314; 315; 316; 317; 318; 319; 27; 320). Theoretically, concurrent use of garlic with other antioxidants may have additive effects.
  • AntiparasiticsAntiparasitics: The antiparasitic effects of garlic have been reviewed (321). It is indicated that garlic oil has broad-spectrum activity against Trypanosoma, Plasmodium, Giardia, Leishmania, and Cochlospermum planchonii. In animal models, allicin decreased Plasmodium infections (322; 323; 324; 325). In animal study, garlic extract augmented Leishmania engulfment and destroys amastigotes by macrophages (323). In vitro, ajoene inhibited enzymes found in Trypanosoma (326). Also, in vitro, garlic had antigiardial activity (327).
  • Cytochrome P450-metabolized herbs and supplementsCytochrome P450-metabolized herbs and supplements: In human study, garlic oil reduced cytochrome P450 2E1 activity (143; 144). In human study, allicin reduced the metabolism of omeprazole by inhibition of CYP2C19 in individuals with the CYP2C19*1/CYP2C19*1 and CYP2C19*1/CYP2C19*2 or *3 genotypes, but not in those with the CYP2C19*2/ CYP2C19*2 genotype (145). Allicin has been found to reduce the effectiveness of cyclosporine by increasing CYP3A4 activity (146). Although animal and human study suggests possible induction or inhibition of various P450 enzymes (145; 143; 147; 148), other study has found garlic to have a lack of an effect on the metabolism of CYP450 substrates (alprazolam, P450 3A4 or dextromethorphan, P450 2D6) (149).
  • Fertility herbs and supplementsFertility herbs and supplements: Based on animal evidence, chronic garlic ingestion for 70 days may be associated with suppression of spermatogenesis (251). Theoretically, concurrent use may modulate the effectiveness of fertility herbs and supplements.
  • Fish oil, EPA (eicosapentaenoic acid)Fish oil, EPA (eicosapentaenoic acid): EPA is found in deep-sea fish oils. Garlic may potentiate antithrombotic effects of EPA, and theoretically, concomitant use of these agents may increase the risk of bleeding. Garlic and fish oil may have additive lipid-lowering effects. In humans, the combination of fish oil and garlic resulted in a reduction of total cholesterol and LDL cholesterol by 20% and 21%, respectively (155).
  • Herbs/supplements used in hematology and blood disordersHerbs/supplements used in hematology and blood disorders: In theory, the risk of bleeding may be increased by concomitant use of garlic and hematologic agents. Bleeding has been associated with garlic use in several studies and case reports, including intraoperatively, possibly related to impaired platelet aggregation or increased fibrinolysis (93; 94; 95; 96; 97; 98; 99; 95; 100; 101; 35; 102; 103; 104; 105; 106). In contrast, there are several low-quality reports that suggest a lack of an effect on ex vivo platelet aggregation (176) or bleeding or hemorrhaging in patients on warfarin anticoagulation therapy (256). Additionally, in human study, coadministration of warfarin and garlic did not alter the pharmacokinetics and pharmacodynamics of warfarin (256; 282).
  • HypoglycemicsHypoglycemics: The available evidence suggests that garlic does not lower blood glucose levels in humans. Although animal studies have reported that garlic or its constituents (such as S-allyl cysteine sulfoxide [SACS]) may decrease glucose concentrations and increase insulin secretion (107; 108), multiple human trials have failed to demonstrate significant effects of oral garlic preparations on measures of glycemic control in diabetic or nondiabetic patients (113; 171; 105; 122; 123; 124; 246; 128; 130; 245). One poor-quality study noted a small significant reduction in mean blood glucose levels in patients treated with 800mg of daily dehydrated garlic (Kwai®), from 89mg/dL to 79mg/dL over four weeks (127). Although the available studies have been small (<100 subjects) with methodological weaknesses, it appears that garlic likely does not exert clinically relevant effects on glucose levels. Evaluation of possible interactions with hypoglycemics is lacking.
  • HypotensivesHypotensives: Numerous studies have reported small mean reductions in systolic and diastolic blood pressure associated with the use of oral garlic vs. placebo, and combination use with antihypertensives may result in additive effects (2; 112; 113; 114; 115; 116; 105; 117; 118; 119; 120; 121; 122; 123; 124; 124; 125; 126; 127; 102; 128; 129; 130; 131; 132; 133; 134; 135; 7; 136; 137; 3; 138; 139; 140; 141; 142). In general, mean differences have been less than 10mmHg (<10%), and the majority of studies have been small (<100 subjects), with poor descriptions of methodology and results.
  • ImmunosuppressantsImmunosuppressants: Based on laboratory and animal study, garlic extract may enhance immune function (288; 289; 290; 291; 292; 293; 294; 288; 289; 290; 291; 292; 293). Theoretically, concurrent use may modulate the effectiveness of immunosuppressants.
  • Intraocular pressure-altering herbsIntraocular pressure-altering herbs: Based on animal study, S-allylmercaptocysteine, a garlic-derived compound, reduced intraocular pressure and may involve the elevation of ANP levels (49). Theoretically, concurrent use may modulate the effectiveness of intraocular pressure-altering herbs.
  • Neurologic herbs and supplementsNeurologic herbs and supplements: In animal models, aged garlic extract prevented deterioration of hippocampal-based memory tasks (296) and had antiamyloidogenic effects (14). In vitro, S-allyl-L-cysteine protected against amyloid beta- and tunicamycin-induced neuronal death (297). The protein TRPA1 mediates the response to pungent irritants found in garlic and is found mainly in nociceptive neurons of peripheral ganglia and in all the mechanosensory epithelia of the inner ear (298).
  • Osteoporosis herbs and supplementsOsteoporosis herbs and supplements: Based on animal study, garlic may suppress bone loss owing to estrogen deficiency (69; 70). Theoretically, concurrent use may modulate the effectiveness of osteoporosis herbs and supplements.
  • Performance-enhancing herbs and supplementsPerformance-enhancing herbs and supplements: Single administration of garlic resulted in increased endurance performance likely due to increase in fibrinolytic activity in the resting state (178). Theoretically, concurrent use may modulate the effectiveness of performance-enhancing herbs and supplements.
  • PhytoestrogensPhytoestrogens: Based on animal study, garlic may have phytoestrogenic activity (71). Theoretically, concurrent use with other phytoestrogens may have additive effects.
  • PotassiumPotassium: Garlic is a source of potassium (299) and may elevate potassium levels. Theoretically, concurrent use of garlic and potassium supplements may increase the risk of hyperkalemia.
  • Pycnogenol®Pycnogenol®: Garlic and Pycnogenol® have been shown to increase human growth hormone secretion in laboratory experiments (328).
  • SeleniumSelenium: Garlic has been found to contain selenium in various studies (329; 330; 331). Garlic supplementation may increase amounts of selenium in the body.
  • Vasodilator herbs and supplementsVasodilator herbs and supplements: In animal and human study, garlic and its major constituent, allicin, had significant vasodilator activity via hyperpolarization of vascular smooth muscle (151; 152; 153; 154). In animal study, diallyldisulfide and allyl mercaptan, metabolites of allicin, did notpossess this vasodilatory action (154). Theoretically, concurrent use of garlic and other vasodilators may have additive effects.
  • ZincZinc: In animal model, a complex of zinc and garlic-derived allixin exhibited high insulin-mimetic and hypoglycemic activity (332). Theoretically, concurrent use may have additive effects.

Garlic/Food Interactions:
  • ButterButter: Garlic oil prevented a fall in fibrinolytic activity due to increased butter consumption (333).
  • OnionOnion: A combination of garlic and onion may lower cholesterol in humans (334).

Garlic/Lab Interactions:
  • Blood glucoseBlood glucose: The available evidence suggests that garlic does not significantly lower blood glucose levels in humans. Although animal study has reported that garlic may decrease glucose concentrations and increase insulin secretion (107; 108), multiple human trials have failed to demonstrate significant effects of oral garlic preparations on measures of glycemic control in diabetic or nondiabetic patients (113; 171; 105; 122; 123; 124; 246; 128; 130; 245), with the exception of one study of suboptimal quality that noted a small, significant reduction in mean blood glucose levels in patients treated with 800mg of daily dehydrated garlic (Kwai®), from 89mg/dL to 79mg/dL over four weeks (127).
  • Blood pressureBlood pressure: Based on numerous clinical studies, garlic may reduce blood pressure (7; 2; 3; 118; 335; 336).
  • Coagulation panelCoagulation panel: An increase in a previously stabilized internationalized ratio (INR) with concomitant garlic and warfarin (Coumadin®) use has been reported, which has been subsequently debated due to limited clinical information (254; 96). However, in a small controlled study, no significant change in INR values was found in a group of patients stabilized on warfarin therapy (INR target 2-3) who were started on 1200mg of aged garlic extract for two months, compared to a placebo group. Clinical outcomes such as increased bleeding were not assessed, and the study was likely too small and brief to significantly measure such outcomes (255).
  • EstrogensEstrogens: Based on animal study, garlic may have phytoestrogenic activity (71) and may increase estrogen levels.
  • FolateFolate: Based on human study, garlic supplementation increased folate levels (337).
  • Homocysteine levelsHomocysteine levels: Based on secondary information, garlic may lower homocysteine levels in the blood.
  • Lipid profileLipid profile: Multiple trials have demonstrated modest lipid-lowering properties of oral garlic supplementation, including decreases in total cholesterol, low-density lipoprotein (LDL), and triglycerides (2; 3; 5; 6; 4). Results have been inconsistent regarding effects of garlic on levels of high-density lipoprotein (HDL), although a meta-analysis of randomized controlled trials did not demonstrate a statistically significant effect when data were pooled (2).
  • Liver-function tests (LFTs)Liver-function tests (LFTs): In animal study, garlic elevated the activity of gamma-glutamate transpeptidase (GGT), glutathione S-transferase (GST), 5'-nucleotidase, alkaline phosphatase (ALP), aspartate transaminase (AST), and alanine transaminase (ALT) (338).
  • PotassiumPotassium: Garlic is a source of potassium (299) and may elevate potassium levels.
  • Urine allylmercapturic acidUrine allylmercapturic acid: Garlic tablets and fresh garlic may result in urinary excretion of allylmercapturic acid (N-acetyl-S-allyl-L-cysteine), which may interfere with urinary monitoring of workers for industrial exposure to allyl halides (339).

Copyright © 2011 Natural Standard (www.naturalstandard.com)


The information in this monograph is intended for informational purposes only, and is meant to help users better understand health concerns. Information is based on review of scientific research data, historical practice patterns, and clinical experience. This information should not be interpreted as specific medical advice. Users should consult with a qualified healthcare provider for specific questions regarding therapies, diagnosis and/or health conditions, prior to making therapeutic decisions.

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