Garlic

Efficacy
Gastric cancer: Four SRs (including 10-27 mixed study designs) found garlic intake associated with a reduced risk of gastric cancer. A fifth SR (n =18 mixed study designs) found that high garlic consumption is associated with reduced gastric cancer risk.

Prostate cancer, breast cancer, and cancers of the upper digestive tract: Three SR (n=9, n=17, n=25 mixed study designs) found a significant reduction in the risk of prostate cancer, breast cancer, and cancers of the upper digestive tract (oral cavity, pharynx, and oesophagus).

Safety
The safety of garlic has not been systematically assessed.  Serious adverse events are not to be expected at recommended doses or normal levels of dietary intake, but herb-drug interactions might occur.

Read the full version of the Garlic summary.

Selenium

Efficacy
Liver and pancreatic cancer: One SR (n=37 mixed study designs) reported a positive association between high-dose selenium intake and reduced liver and pancreatic cancer risk. It is also found that at intake ≥55 μg/day,  a decreased risk was observed in oesophagus, liver, and pancreatic cancer.

Safety
Selenium is generally considered safe. However, the safety of the therapy has not been systematically assessed. It has been associated with risks related to over-supplementation. Chronic selenium poisoning has been reported. Long-term supplementation may increase the risk of type 2 diabetes.

Read the full version of the Selenium summary.

Garlic

Efficacy
Colorectal cancer (CRC): The evidence is contradictory on the effect of  garlic on reducing the risk of CRC. Six SRs have been conducted assessing the effect of garlic consumption on CRC. Three of the SRs (n=5, n=14, n=16 cohort & case-control studies) found no significant association between garlic intake and CRC. Two SRs (n=9, n=11 cohort & case-control studies) found positive associations between garlic intake and reduced CRC risk. One SR (n= 13 cohort & case-control studies) found garlic intake associated with an increased risk of CRC.

Safety
The safety of garlic has not been systematically assessed.  Serious adverse events are not to be expected at recommended doses or normal levels of dietary intake, but herb-drug interactions might occur.

Read the full version of the Garlic summary.

Green tea

Efficacy
The evidence of effect of green tea on reducing cancer risk is inconclusive due to low methodological quality and inconsistent results. These findings are based on a Cochrane review (n=142 mixed study designs: 11 RCTs and 131 nonexperimental studies)

High green tea consumption reduced the risk for certain cancers (colon, colorectal, endometrial, gynaecological, oral, ovarian, and prostate); however, all studies reported low certainty of the evidence. 

Green tea supplementation: the Cochrane SR found a decreased risk among prostate cancer patients (n=3 RCTs), an increased risk among gynaecological cancer patients (n=2 RCTs) and no evidence of effect among non-melanoma skin cancer patients (n=1 RCT). For all three, the evidence was of low-certainty.

Safety
Green tea prevention is generally safe with moderate, regular, and habitual use, with only transient and mild adverse events. Adverse effects reported include various gastrointestinal problems, effects on liver enzymes, and allergic reactions. Liver-related adverse effects have been reported, particularly with high intakes or in those with existing liver disease.

Read the full version of the Green tea summary.

Garlic

Efficacy
Other cancers: The evidence suggests garlic is not effective in reducing the risk of different cancer types. Two other SRs (n=14, n=22 mixed study designs) conducted on different cancer types found no effect of garlic.

Safety
The safety of garlic has not been systematically assessed.  Serious adverse events are not to be expected at recommended doses or normal levels of dietary intake, but herb-drug interactions might occur.

Read the full version of the Garlic summary.

Selenium

Efficacy
Lung cancer: The evidence suggests that selenium is not effective in reducing lung cancer risk. Two SRs (n=37, n=81 mixed study designs) found that selenium did not reduce lung cancer risk. A third SR (n=15 observational studies) found that low levels of selenium are highly correlated to lung cancer.

Prostate cancer: The evidence suggests selenium is not effective on reducing risk of prostate cancer. Two SRs (n=37, n=81 mixed study designs) found that prostate cancer risk was not reduced by selenium. On the contrary, studies showed a high incidence of high-grade prostate cancer among those taking selenium supplementation. A case-control study (n=909) also concluded no association between selenium and reduced prostate cancer risk.  

Various cancers: The evidence suggests selenium is not effective in reducing the risk of bladder, breast, colorectal, haematological, oesophageal, skin, or stomach cancer. Two SRs (n=37, n=81 mixed study designs) found that selenium did not reduce the risk of getting any of the latter cancers.  

Safety
Selenium is generally considered safe. However, the safety of the therapy has not been systematically assessed. It has been associated with risks related to over supplementation. Chronic Selenium poisoning has been reported. Symptoms include hair loss, thickened nails, nausea, vomiting, fatigue, and paraesthesia and paralysis. Long-term supplementation may increase the risk of type 2 diabetes.

Read the full version of the Selenium summary.