Written by Edzard Ernst and the CAM-Cancer Consortium.
Updated August 14, 2013

Garlic (Allium sativum)

Is it safe?

Adverse effects

Garlic (Allium sativum) is generally considered to be non-toxic 1,2,47,48. Adverse effects that have been documented in humans include a burning sensation in the mouth and gastrointestinal tract, nausea, diarrhea, vomiting 2,47 and body odour. The allergenic potential of garlic is well recognized, and the allergens have been identified as diallyl disulfide, allylpropyl sulfide and allicin 2,5. Preclinical data on chronic toxicity are conflicting 2,49. High doses are stated to cause anaemia due to both decreased haemoglobin synthesis and haemolysis 2,50. Genotoxicity studies using the micronucleus test have reported both positive and negative findings 2. No evidence of mutagenicity has been reported when assessed using the Ames and Ree assay 2.

Contraindications

Allergy to Allium species, peptic ulcer, there may be an increased risk of bleeding with the use of garlic supplements in patients undergoing surgery 2.

Interactions

Garlic intake might increase the effects of anticoagulants. A potential interaction between garlic and warfarin has been documented 2,51.Garlic extracts are unlikely to alter the metabolism of drugs primarily dependent on the CYP2D6 or CYP3A4 pathway 52,53. However, in patients carrying a certain CYP3A5 allele, garlic could affect the clearance of docetaxel, leading to higher toxicity 52. Findings from in-vitro studies further suggest, that garlic suppresses the expression and activity of the CYP P450 subtype 2C9 (pharmacokinetics possibly affected: e.g. cyclophosphamid, diclofenac, haloperidol, ibuprofen, naproxen, paclitaxel, piroxi¬cam, and tamoxifen) 54. Slight changes were also found in CYP1A2 levels (increase) and CYP2E1 activity (decrease) (pharmacokinetics possibly affected: e.g. ondansetron, etoposide) 55.

An increase has been noted in the expression of duodenal P-glycoprotein after the ingestion of garlic extracts 56. This interaction is thought to be the most probable mechanism for the known impact that garlic supplements exert on the first-pass metabolism of HIV protease inhibitors 57-59.

Finally, alterations in the activity of the phase II biotransformation enzymes UDP glucuronosyltransferase and glutathion-S-transferase have been observed after the ingestion of garlic extracts 55.

Quality issues

Garlic preparations should adhere to GAP, GMP and in Europe to the European guidelines “Quality of Herbal Medicinal products”.

Warning

Insufficient information exists for garlic use beyond amounts consumed as part of the daily diet during pregnancy and lactation. Garlic is reputed to act as an abortifacient and to affect the menstrual cycle, and is also reported to be utero-active 2. There are only few clinical studies during pregnancy and lactation. However, there are not experimental or clinical reports on adverse effects during pregnancy or lactation 2. In view of this, doses of garlic greatly exceeding amounts used in foods should not be taken during pregnancy and lactation 2.

Other problems

Overdose may cause nausea and vomiting.

Citation Edzard Ernst, CAM-Cancer Consortium. Garlic (Allium sativum) [online document]. http://ws.cam-cancer.org/The-Summaries/Herbal-products/Garlic-Allium-sativum. August 14, 2013.

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