Garlic

Purported Benefits, Side Effects & More
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This information describes the common uses of Garlic, how it works, and its possible side effects.
Tell your healthcare providers about any dietary supplements you’re taking, such as herbs, vitamins, minerals, and natural or home remedies. This will help them manage your care and keep you safe.

What is it?

Evidence on whether garlic can help lower cholesterol levels or blood pressure are mixed.

Derived from the bulb or clove of the plant, garlic is used as a spice worldwide. It is also used traditionally for a variety of conditions such as maintaining circulatory function, to treat infections, and to boost the immune system.

Studies on whether garlic can reduce cholesterol or lower blood pressure are mixed, but other data suggest it may help reduce some cardiovascular risk factors.

Processing may reduce active compounds found in garlic products. For example, garlic powder and garlic essential oil do not contain allicin or ajoene, compounds believed to have cholesterol-reducing and blood-thinning properties.

Patients taking warfarin or other blood thinners should ask their doctor before taking garlic supplements.

What are the potential uses and benefits?

  • To treat cardiovascular disease
    Studies have shown mixed results, but garlic may help reduce some risk factors for cardiovascular disease. Patients should ask their doctor before taking garlic supplements.
  • To prevent and treat cancer
    Evidence on whether garlic can reduce risk of gastric and colorectal cancers is mixed.
  • To treat infections
    Data suggest garlic can stimulate the immune system, but it is unclear whether it can treat infections.

What are the side effects?

  • Headache
  • Fatigue
  • Offensive odor, bad breath
  • Stomach upset
  • Diarrhea
  • Changes in natural intestinal bacteria
  • Sweating
  • Low blood sugar
  • Altered platelet function
  • Increased bleeding risk

Case reports (Oral)

  • Bleeding, reduced blood clotting: Following excessive use of garlic supplements.
  • Redetected levels of HIV: In 2 cases due to interactions between garlic and darunavir. The effectiveness of the medication returned some time after garlic intake was stopped.
  • Chemical burns in the mouth lining: Following consumption of crushed garlic.
  • Allergic reactions: Two cases, following ingestion of garlic.
  • Liver toxicity: After a liver transplant patient was started on high-dose garlic supplementation to treat low levels of oxygen in the blood.

Case reports (Topical)

  • Chemical burns: Multiple cases from the topical use of garlic in attempts to treat warts, rash/itching, tooth pain, acne, and sore throat.
  • Skin rash: Due to “garlic necklace” on infant’s neck to treat nasal congestion

What else do I need to know?

Patient Warnings:

  • Garlic supplements should be discontinued 1–2 weeks before surgery because of the potential for increased bleeding.
  • Topical use of garlic preparations should be avoided. Chemical burns have occurred in attempts to treat a variety of conditions.

Do Not Take if:

  • You are taking warfarin or other blood thinners: Garlic may increase the risk of bleeding or bruising.
  • You take cyclosporine: Garlic can reduce its effectiveness and potentially cause transplant rejection.
  • You are taking protease inhibitors (darunavir/Prezista®, saquinavir/Fortovase®, Invirase®): In 2 cases of HIV patients who regularly consumed garlic, darunavir became ineffective. Other studies found garlic reduced blood levels of saquinavir and may reduce its effectiveness.
  • You are taking insulin: Dose adjustments may be required from potential blood sugar-lowering effects with garlic supplements. Take with caution and consult your doctor.
  • You are taking drugs that are substrates of CYP450 2C9, 2C19, or 3A4: Garlic may increase the risk of side effects of these drugs.
  • You are taking drugs that are substrates of P-glycoprotein: Garlic may reduce the activity of such drugs.