Adverse Effects and Toxicity

Garlic is most commonly consumed as a food, rather than as a supplement. According to the Food and Drug Administration (FDA), chopped garlic and oil mixes left at room temperature have the ability to result in fatal botulism food poisoning (84). Such products need to be kept refrigerated, especially those that do not contain acidifying agents such as phosphoric or citric acid. Clostridium botulinum bacteria are dispersed throughout the environment, but are not dangerous in the presence of oxygen. The spores produce a deadly toxin in anaerobic, low-acid conditions. The garlic-in-oil mixture provides the environment for the spores to produce their toxin, leading to botulism. At least 40 cases of this poisoning were reported in the late 1980s.

Since the effects of garlic as a medicinal agent have been studied, reports of common side effects have been reported. The most common of which is malodorous breath and body odor. This effect generally can last many hours after garlic consumption and is not removed by brushing teeth or bathing. One study attempted to reveal the mechanism behind this unpopular effect. Air from the mouth and lungs, as well as urine samples were analyzed for sulfur-containing gases (hydrogen sulfide, methanethiol, allyl mercaptan, allyl methyl sulfide, allyl methyl disulfide, and allyl disulfide) after garlic ingestion. Most of the gas levels were present in higher levels in mouth air than lung air or urine up to 3 hours after ingestion and decreased thereafter. However, allyl methyl sulfide concentrations remained high in mouth and lung air, and urine. This indicates that this gas was absorbed and released from the lungs and in the urine. The authors concluded that systemic absorption of allyl methyl sulfide was responsible for the prolonged odor caused by garlic consumption and therefore explained why oral hygiene could not abolish the smell (85).

Many of the cardiovascular trials reported side effects of garlic use, with the most frequently reported being GI symptoms and garlic breath. In addition, rash and prolonged oozing from a razor cut were reported in one of these studies (86). Other commonly described side effects associated with garlic use include GI effects such as abdominal pain, fullness, anorexia, and flatulence.

Coagulation dysfunctions have also been reported, such as postoperative bleeding and prolonged clotting time (87). One case of spinal epidural hematoma associated with excessive garlic ingestion has been reported. An 87-year-old man who reported to consume an average of four gloves of garlic per day to prevent heart disease, presented to the emergency room with acute onset abdominal discomfort and bilateral sensory and motor paralysis in the lower extremities. Prothrombin time was 12.7 seconds and partial thromboplastin time was 22.3 seconds. The patient had no additional risk factors for bleeding and was taking no other medications that would affect bleeding tendency. The platelet inhibition caused by garlic was determined to be the cause (88).

In 2001, Hoshino et al. (89) investigated whether different garlic preparations have undesirable effects on the GI mucosa in dogs. When administered directly to the stomach, AGE did not produce any changes compared to control to the mucosa, whereas boiled garlic powder caused redness, and raw garlic powder caused redness and erosion of the mucosa. Pulverized enteric-coated tablets caused redness and a loss of epithelial cells. Although these findings were significant, further study in humans should be done to confirm the relevance of these findings.

In addition, garlic has been shown to change the odor of breast milk in lactating women, as well as alter the sucking patterns of nursing infants (ref. 80; see Subheading 7.2).

Reports of adverse effects in garlic studies are inconsistent. Studies using AGE have reported fewer side effects and toxicities than those using other garlic preparations (5). Therefore, the frequency and severity of effects seen with garlic may vary with the type of preparation used.

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