Serum cholesterol elevation is related to an increased risk for coronary artery disease. Nonpharmacologic interventions and lipid-lowering drugs are available for primary and secondary prevention of coronary heart disease. Pharmacologic therapies are associated with adverse effects, and finding an effective yet harmless therapy is a high priority. Recent meta-analyses of garlic's efficacy in lowering cholesterol levels have demonstrated an average decrease of 9 to 12 percent when compared with placebo although re-analysis of some of the data demonstrated diminished efficacy.

Stevinson and associates reevaluated the specific effect of garlic on total serum cholesterol level in persons with elevated levels by searching the literature and evidence-based data collections. Selected studies included these criteria: randomized, double-blind and placebo-controlled; used garlic monopreparations; included patients with a mean total cholesterol level of at least 200 mg per dL (5.20 mmol per L); and reported total cholesterol level as an end point. Thirteen trials met the inclusion criteria and provided data satisfactory for pooling. A total of 796 persons were involved. Ten trials reported differences that favored garlic over placebo. Meta-analysis of all of the trials indicated a significant difference (equivalent to a 5.8 percent reduction) in the reduction of total cholesterol level from baseline in favor of garlic compared with placebo. Further analysis of the five studies that presented other lipid data showed non-significant differences in the reduction of low-density lipoprotein cholesterol and nonsignificant differences in the increase of high-density lipoprotein cholesterol between garlic and placebo. In the 10 studies that examined adverse effects, the most common were gastrointestinal symptoms and garlic breath.

Compared with conventional methods of lowering lipid levels, the estimated reduction for garlic is unimpressive. Dietary interventions have been demonstrated to lower total cholesterol levels by 5.3 percent after six months compared with no treatment. Statin drugs can achieve a reduction of total cholesterol between 17 percent and 32 percent. The consistency of the active ingredient preparation in the formulations of garlic used in the studies is also questionable. None of the studies was long enough to provide useful data on any association between garlic consumption and important clinical outcomes.

The authors conclude that garlic is superior to placebo in lowering elevated total cholesterol levels. The size of the effect, however, is modest, and the dependability of the effect is debatable. In clinical practice, this finding means that garlic is not an efficient way to decrease the total serum cholesterol level. Patients should be advised that, according to current evidence, any specific effect of garlic on the cholesterol level is small and may not be clinically meaningful.