Garlic (Allium sativum)

Traditional texts describe the preparations below, and where those texts give specific amounts, the amounts are quoted here as part of that record. This is a record of historical practice, not a dosage recommendation - the figures are what the sources say, not what HerbMate tells you to take. Amounts, potency, and safe use vary enormously by preparation, plant part, and person.

Case reports suggest that garlic may have additive blood pressure-lowering effects with lisinopril, and may cause bleeding in those taking warfarin or fluindione. It has also been suggested that any antiplatelet effects of garlic may be additive with conventional antiplatelet drugs and NSAIDs, and studies suggest that garlic may reduce isoniazid levels. However, no interaction has been proven with any of these drugs. In general, garlic seems to have no effect, or have only clinically irrelevant effects when it is given with alcohol, benzodiazepines (such as midazolam), caffeine, chlorzoxazone, dextromethorphan, docetaxel, gentamicin, paracetamol (acetaminophen), rifampicin (rifampin) or ritonavir. Any interaction between garlic and fish oils may be beneficial. One study suggested that a high-fat diet did not affect the absorption of some of the active constituents of garlic oil.

Protease inhibitors

A garlic supplement reduced the plasma levels of saquinavir in one study, but had little effect in another. Another garlic supplement did not significantly affect the pharmacokinetics of a single dose of ritonavir. In a study in 9 healthy subjects garlic reduced the AUC and maximum and minimum plasma levels of saquinavir by about 50%. The garlic was taken in the form of a dietary supplement (GarliPure, Maximum Allicin Formula caplets) twice daily for 20 days. Saquinavir 1.2 g three times daily was given for 4-day periods before, during and after the garlic supplement. Fourteen days after the garlic supplement was stopped the saquinavir pharmacokinetics had still not returned to baseline values. Of the 9 subjects, 6 had a substantial drop in the AUC of saquinavir while taking garlic, then a rise when garlic was stopped. The remaining 3 had no change in the AUC of saquinavir while taking garlic, but had a drop when garlic was stopped. However, in another study, garlic extract (Garlipure) 1.2 g daily for 3 weeks had no significant effect on the pharmacokinetics of a single 1.2-g dose of saquinavir (a slight decrease in AUC in 7 subjects and a slight increase in 3). In a study in 10 healthy subjects the use of a garlic extract (10 mg, equivalent to 1 g of fresh garlic) twice daily for 4 days did not significantly affect the pharmacokinetics of a single 400-mg dose of ritonavir. There was a non-significant 17% decrease in the AUC of ritonavir. The garlic was given in the form of capsules (Natural Source Odourless Garlic Life Brand). Gastrointestinal toxicity was noted in 2 patients taking garlic or garlic supplements when they started to take ritonavir-containing regimens.

ACE inhibitors

In a single report, a patient taking lisinopril developed marked hypotension and became faint after taking garlic capsules.

Warfarin and related drugs

An isolated report described increases in the anticoagulant effects of warfarin in two patients taking garlic supplements. Another report described a decrease in anticoagulant effects of fluindione in a patient taking garlic tablets. Garlic supplements alone have also rarely been associated with bleeding. However, in one study, aged garlic extract did not increase the INR or risk of bleeding in patients taking warfarin.

Herbal medicines; Fish oil

Garlic supplements and fish oils may have beneficial effects on blood lipids. In a placebo-controlled study in 46 subjects with moderate, untreated hypercholesterolaemia, combined use of garlic pills 300 mg three times daily (Kwai) and fish oil capsules 4 g three times daily for 12 weeks was compared with either garlic or fish oil alone. Garlic modestly reduced total cholesterol, and fish oil did not alter this effect. Fish oil reduced triacylglycerol levels, and garlic did not alter this effect. Garlic alone reduced low-density-lipoprotein cholesterol, and combined use with fish oil reversed the increase of low-density-lipoprotein cholesterol seen with fish oil alone and produced a reduction similar to that seen with garlic alone. Slight reductions in blood pressure were also reported with all treatments. The fish oil used was 1-g capsules (Nupulse) each containing eicosapentaenoic acid 180mg and docosahexaenoic acid 120 mg.

Food

The information regarding the use of garlic with food is based on experimental evidence only.

Dextromethorphan

Garlic does not appear to affect the pharmacokinetics of dextromethorphan or debrisoquine. A study in 14 healthy subjects found that Kwai garlic tablets 600 mg twice daily for 14 days did not affect the pharmacokinetics of a single 30-mg dose of dextromethorphan. Garlic oil 500 mg three times daily for 28 days did not affect the metabolism of debrisoquine 5 mg in young or elderly healthy subjects.

Benzodiazepines

Garlic does not appear to affect the pharmacokinetics of alprazolam, midazolam or triazolam to a clinically relevant extent. A study in 14 healthy subjects found that Kwai garlic tablets 600 mg twice daily for 14 days did not affect the pharmacokinetics of a single 2-mg dose of alprazolam. Similarly, garlic oil 500 mg three times daily for 28 days did not affect the metabolism of midazolam 8 mg in young or elderly healthy subjects.

Antiplatelet drugs

Garlic may have antiplatelet properties. It might therefore be expected to increase the risk of bleeding with conventional antiplatelet drugs and other drugs that have antiplatelet adverse effects. In a study in 23 healthy subjects, liquid aged garlic extract 5mL (Kyolic), given daily for 13 weeks, inhibited both the rate of platelet aggregation and total platelet aggregation. Similar effects were found in another study in 28 healthy subjects given aged garlic extract capsules 2.4 g, 4.8 g and 7.2 g. Each dose was given daily for a 6-week period.

Gentamicin

The information regarding the use of garlic with gentamicin is based on experimental evidence only.

Docetaxel

Garlic does not appear to affect the pharmacokinetics of intravenous docetaxel. localised, breast cancer were given 1-hour intravenous infusions of docetaxel 30 mg/m2 weekly for 3 weeks (days 1, 8 and 15). Five days after the first infusion, garlic tablets 600mg were taken twice daily for 13 days (days 5 to 17). The garlic tablets used were GarliPure Maximum Allicin Formula, Natrol, containing 3.6mg of allicin per tablet. Patients were also given a premedication regimen of oral dexamethasone 8 mg 12 hours before each docetaxel infusion and then every 12 hours for two more doses, and ondansetron 8 mg, ranitidine 150 mg and diphenhydramine 25 mg half an hour before each infusion of docetaxel. Garlic tablets had no effect on the pharmacokinetics of docetaxel on the second or third week, when compared with the first week (i.e. after 4 and 12 days’ use of garlic).

Herbal medicines; Caffeine containing

Garlic did not interact with caffeine and is therefore unlikely to interact with caffeine-containing herbs, as a result of this constituent.

Chlorzoxazone

The metabolism of chlorzoxazone is modestly inhibited by garlic but this effect is probably not clinically relevant. Garlic oil 500 mg, given to 12 healthy subjects three times daily for 28 days, reduced the conversion of a single 500-mg dose of chlorzoxazone to 6-hydroxychlorzoxazone by about 40%. In a later similar study by the same authors, in 12 elderly healthy subjects, a smaller reduction of 22% was seen. Another study in 8 healthy subjects found that a high dose of the garlic constituent diallyl sulfide 200 micrograms/kg (equivalent to 15 cloves of fresh garlic, containing 1 mg/g diallyl sulfide), reduced the conversion of chlorzoxazone to 6-hydroxychlorzoxazone by about 30%.

Alcohol

The interaction between garlic and alcohol is based on experimental evidence only.

Isoniazid

The interaction between garlic and isoniazid is based on experimental evidence only.

Rifampicin (Rifampin)

The information regarding the use of garlic with rifampicin is based on experimental evidence only.

Paracetamol (Acetaminophen)

Studies in healthy subjects found that garlic did not affect the pharmacokinetics of single-dose paracetamol to a clinically relevant extent. A study in 16 healthy subjects found that the use of an aged garlic extract (approximately equivalent to 6 to 7 cloves of garlic daily) for 3 months had little effect on the metabolism of a single 1-g oral dose of paracetamol.

Caffeine

Garlic does not appear to affect the pharmacokinetics of caffeine. Garlic oil 500 mg three times daily for 28 days did not affect the metabolism of a single 100-mg dose of caffeine in young or elderly healthy subjects.

These entries keep the wording of the reference they come from, and three phrases mean three different things. "No interactions found" means the source looked for reports of that pairing and found none. "No data found" means nothing has been published either way. "Based on experimental evidence only" means the effect has been seen in a laboratory or animal study but not in people - a reason for caution, not a report of harm.