Garlic is of the genus Allium, which also includes onions, shallots, leeks, and chives.1 Garlic contains multiple bioactive compounds such as flavonoids, sulfur compounds, and selenium.
Though mechanistic studies conducted primarily in vitro suggest that individual components, particularly the sulfur compounds, or extracts of garlic have anticancer properties, epidemiologic studies have demonstrated mixed results.
The majority of epidemiologic studies suggest that supplementation with a garlic pill does not affect cancer risk, particularly for colorectal (CRC), gastric, or lung cancer.
For CRC, a meta-analysis of 2 prospective cohort studies found that garlic supplements were not associated with incidence of CRC compared with nonuse, though there was a nonsignificant trend toward an increased risk of the disease (relative risk [RR], 1.24; 95% CI, 0.99-1.54).2 Another meta-analysis, which included 4 cohort studies, also demonstrated no association between garlic supplement use and CRC compared with nonuse (RR, 1.12; 95% CI, 0.96-1.31).3 Both meta-analyses had no heterogeneity. A meta-analysis that included 5 cohort studies demonstrated a marginally increased risk of CRC with garlic supplements compared with nonuse (RR, 1.18; 95% CI, 1.02-1.36).4
An analysis of the prospective VITAL cohort study demonstrated no effect on risk of lung cancer with garlic supplement use compared with nonuse.5
Several randomized controlled trials evaluated the relationship between garlic supplementation or extract and cancer. A small trial randomly assigned 51 patients with colorectal adenomas to high-dose aged garlic extract (AGE) or low-dose AGE as a control. After 1 year of treatment, patients taking high-dose AGE demonstrated a significantly lower number and size of colorectal adenomas compared with patients who received the low-dose AGE (P < .05 and P = .04, respectively).6
Another randomized, controlled trial included 3365 individuals aged 35 to 64 from China with baseline endoscopies, each of whom was assigned to receive amoxicillin and omeprazole for 2 weeks, vitamin C, E, and selenium for 7.3 years, or AGE and steam-distilled garlic oil for 7.3 years.7 After 4 and 8 years of follow-up, the risk of severe chronic atrophic gastritis, intestinal metaplasia, dysplasia, or gastric cancer was significantly lower with conventional Helicobacter pylori treatment (amoxicillin and omeprazole; OR, 0.77; 95% CI0.62-0.95 and OR, 0.60; 95% CI, 0.47-0.75), but not garlic supplementation. These findings were confirmed after 15 years of follow-up, though garlic supplementation resulted in a nonsignificant trend of reduced esophageal or gastric cancer (HR, 0.62; 95% CI, 0.37-1.05).8