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Garlic (Allium sativum) is an herb related to onion, leeks, and chives. It is commonly used for conditions related to the heart and blood system.

Garlic produces a chemical called allicin. This is what seems to make garlic work for certain conditions. Allicin also makes garlic smell. Some products are made "odorless" by aging the garlic, but this process can also change the effects of garlic.

People commonly use garlic for high blood pressure, high levels of cholesterol or other fats in the blood, and hardening of the arteries. It is also used for the common cold, osteoarthritis, and many other conditions, but there is no good scientific evidence to support these uses. There is also no good evidence to support using garlic for COVID-19.

Natural Medicines rates effectiveness based on scientific evidence according to the following scale: Effective, Likely Effective, Possibly Effective, Possibly Ineffective, Likely Ineffective, Ineffective, and Insufficient Evidence to Rate.
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  • Atherosclerosis.  When used alone or in combination with other ingredients, oral garlic seems to slow the progression of atherosclerosis.
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  • Diabetes.  Most clinical research suggests that oral garlic, taken alone or in combination with metformin for at least 12 weeks, modestly reduces fasting blood glucose levels in patients with diabetes. It is unclear if garlic improves postprandial glucose levels or glycated hemoglobin (HbA1c).
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  • Hyperlipidemia.  Most research shows that taking oral garlic for at least 8 weeks seems to modestly reduce total and low-density lipoprotein (LDL) cholesterol levels in patients with hyperlipidemia. However, garlic doesn't seem to improve high-density lipoprotein (HDL) cholesterol or triglyceride levels.
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  • Hypertension.  Some clinical research suggests that oral garlic seems to modestly reduce blood pressure in hypertensive and normotensive patients.
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  • Nonalcoholic fatty liver disease (NAFLD).  Oral garlic seems to reduce the severity of hepatic steatosis in patients with NAFLD.
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  • Periodontitis.  Oral aged garlic extract seems to improve some measures of gum health in people with mild to moderate periodontitis.
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When taken by mouth: Garlic is likely safe for most people. Garlic has been used safely for up to 7 years. It can cause side effects such as bad breath, heartburn, gas, and diarrhea. These side effects are often worse with raw garlic. Garlic might also increase the risk of bleeding and cause allergic reactions in some people.

When applied to the skin: Garlic products are possibly safe. Gels, pastes, and mouthwashes containing garlic have been used for up to 3 months. But garlic might cause skin damage that is similar to a burn. RAW garlic is possibly unsafe when applied to the skin. It might cause severe skin irritation.

Special Precautions & Warnings:

Pregnancy and breast-feeding: Garlic is likely safe to take by mouth in the amounts normally found in food. Garlic is possibly unsafe when used in medicinal amounts during pregnancy and when breast-feeding. There isn't enough reliable information about the safety of applying garlic to the skin if you are pregnant or breast feeding. Stay on the safe side and avoid use.

Children: Garlic is possibly safe when taken by children in doses of up to 300 mg three times daily for up to 8 weeks. There isn't enough reliable information to know if garlic is safe when used in larger doses or for longer than 8 weeks. It is possibly unsafe to apply raw garlic to the skin. It might burn the skin.

Bleeding disorder: Garlic, especially fresh garlic, might increase the risk of bleeding.

Surgery: Garlic might prolong bleeding and interfere with blood pressure. Garlic might also lower blood sugar levels. Stop taking garlic at least two weeks before a scheduled surgery.

Birth control pills (Contraceptive drugs)

Interaction Rating=Moderate Be cautious with this combination.

Some birth control pills contain estrogen. The body breaks down the estrogen in birth control pills to get rid of it. Garlic might increase the breakdown of estrogen. Taking garlic along with birth control pills might decrease the effectiveness of birth control pills. If you take birth control pills along with garlic, use an additional form of birth control such as a condom.
Some birth control pills include ethinyl estradiol and levonorgestrel (Triphasil), ethinyl estradiol and norethindrone (Ortho-Novum 1/35, Ortho-Novum 7/7/7), and others.

Cyclosporine (Neoral, Sandimmune)

Interaction Rating=Moderate Be cautious with this combination.

The body breaks down cyclosporine (Neoral, Sandimmune) to get rid of it. Garlic might increase how quickly the body breaks down cyclosporine (Neoral, Sandimmune). Taking garlic along with cyclosporine (Neoral, Sandimmune) might decrease the effectiveness of cyclosporine (Neoral, Sandimmune). Do not take garlic if you are taking cyclosporine (Neoral, Sandimmune).

Isoniazid (Nydrazid, INH)

Interaction Rating=Major Do not take this combination.

Garlic might reduce how much isoniazid (Nydrazid, INH) the body absorbs. This might decrease how well isoniazid (Nydrazid, INH) works. Don't take garlic if you take isoniazid (Nydrazid, INH).

Medications changed by the liver (Cytochrome P450 2E1 (CYP2E1) substrates)

Interaction Rating=Moderate Be cautious with this combination.

Some medications are changed and broken down by the liver.
Garlic oil might decrease how quickly the liver breaks down some medications. Taking garlic oil along with some medications that are changed by the liver can increase the effects and side effects of your medication. Before taking garlic oil talk to your healthcare provider if you take any medications that are changed by the liver.
Some medications that are changed by the liver include acetaminophen, chlorzoxazone (Parafon Forte), ethanol, theophylline, and drugs used for anesthesia during surgery such as enflurane (Ethrane), halothane (Fluothane), isoflurane (Forane), and methoxyflurane (Penthrane).

Medications changed by the liver (Cytochrome P450 3A4 (CYP3A4) substrates)

Interaction Rating=Moderate Be cautious with this combination.

Some medications are changed and broken down by the liver.
Garlic might increase how quickly the liver breaks down some medications. Taking garlic along with some medications that are broken down by the liver can decrease the effectiveness of some medications. Before taking garlic talk to your healthcare provider if you are taking any medications that are changed by the liver.
Some medications changed by this liver include lovastatin (Mevacor), ketoconazole (Nizoral), itraconazole (Sporanox), fexofenadine (Allegra), triazolam (Halcion), and many others.

Medications that slow blood clotting (Anticoagulant / Antiplatelet drugs)

Interaction Rating=Moderate Be cautious with this combination.

Garlic might slow blood clotting. Taking garlic along with medications that also slow clotting might increase the chances of bruising and bleeding.
Some medications that slow blood clotting include aspirin, clopidogrel (Plavix), diclofenac (Voltaren, Cataflam, others), ibuprofen (Advil, Motrin, others), naproxen (Anaprox, Naprosyn, others), dalteparin (Fragmin), enoxaparin (Lovenox), heparin, warfarin (Coumadin), and others.

Medications used for HIV/AIDS (Non-Nucleoside Reverse Transcriptase Inhibitors (NNRTIs))

Interaction Rating=Major Do not take this combination.

The body breaks down medications used for HIV/AIDS to get rid of them. Garlic can increase how fast the body breaks down some medication for HIV/AIDS. Taking garlic along with some medications used for HIV/AIDS might decrease the effectiveness of some medications used for HIV/AIDS.
Some of these medications used for HIV/AIDS include nevirapine (Viramune), delavirdine (Rescriptor), and efavirenz (Sustiva).

Saquinavir (Fortovase, Invirase)

Interaction Rating=Major Do not take this combination.

The body breaks down saquinavir (Fortovase, Invirase) to get rid of it. Garlic might increase how quickly the body breaks down saquinavir. Taking garlic along with saquinavir (Fortovase, Invirase) might decrease the effectiveness of saquinavir (Fortovase, Invirase).

Warfarin (Coumadin)

Interaction Rating=Moderate Be cautious with this combination.

Warfarin (Coumadin) is used to slow blood clotting. Garlic might increase the effectiveness of warfarin (Coumadin). Taking garlic along with warfarin (Coumadin) might increase the chances of bruising and bleeding. Be sure to have your blood checked regularly. The dose of your warfarin (Coumadin) might need to be changed.

Herbs and supplements that might lower blood pressure: Garlic might lower blood pressure. Taking it with other supplements that have the same effect might cause blood pressure to drop too much. Examples of supplements with this effect include andrographis, casein peptides, L-arginine, niacin, and stinging nettle.
Herbs and supplements that might lower blood sugar: Garlic might lower blood sugar. Taking it with other supplements with similar effects might lower blood sugar too much. Examples of supplements with this effect include aloe, bitter melon, cassia cinnamon, chromium, and prickly pear cactus.
Herbs and supplements that might slow blood clotting: Garlic might slow blood clotting and increase the risk of bleeding. Taking it with other supplements with similar effects might increase the risk of bleeding in some people. Examples of supplements with this effect include ginger, ginkgo, nattokinase, and Panax ginseng.

There are no known interactions with foods.

Garlic has most often been used by adults in doses of 2400 mg by mouth daily for 12 months. Garlic extracts are usually standardized by the amount of allicin they contain. This typically ranges from 1.1% to 1.3%. It's a good idea to look for supplements that are coated (enteric coating) so they will dissolve in the intestine and not in the stomach. Garlic is also used in creams, gels, pastes, and mouthwashes. Speak with a healthcare provider to find out what dose might be best for a specific condition.

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