Results and lessons from clinical trials using dietary supplements for cancer: direct and indirect investigations.

Randomized controlled trials are generally regarded as the standard of study designs to determine potential causality. The inclusion of a placebo group in these trials, when appropriate, is generally needed to access the efficacy of a drug or dietary supplement. The recent increasing use of dietary supplements and herbal medications by patients makes it imperative to reevaluate the past findings of clinical studies. Several large-scale trials of dietary supplements have been tested in various populations to determine their effect on cancer prevention. Other trials have focused on patients already diagnosed with cancer. In the latter case, it is difficult to involve a placebo because of the serious nature of the disease. Nevertheless, much has been gleaned from these trials directly and indirectly. Overall, when analyzing primary endpoints in these trials, the results have been discouraging and even support the nonuse of certain supplements because of potential adverse effects. Other secondary endpoints in these same trials have revealed some potential encouraging and discouraging data. Individuals who currently qualify for the potential use of dietary supplements for cancer may be restricted to those who have a deficiency in a certain compound despite adequate dietary sources or lifestyle changes. Those individuals with a smoking history or other unhealthy lifestyle seem to have the most to gain or lose from taking certain dietary supplements for cancer. The time seems more than ripe to evaluate past adequate trials with supplements, such as beta-carotene, N-acetyl-cysteine, selenium, shark cartilage, vitamin C, vitamin E, and others. Again, these studies have been disappointing, but they provide insight for the clinician and patient of what to potentially expect when using these supplements for cancer. In addition, indirect trials for other conditions (cardiovascular) may provide future insight into possible results for future cancer prevention trials.