Selenium is a component of glutathione peroxidase, which possesses antioxidantactivity, and demonstrates antioxidant properties in humans. Long-term clinical benefits remain controversial.

Keshan disease is a cardiomyopathy (heart disease) restricted to areas of China in people having an extremely low selenium status. Prophylactic administration of sodium selenite has been shown to significantly decrease the incidence of this disorder.Selenium is used to treat and prevent selenium deficiency (for example in those with HIV or receiving enteral feedings).

Initial evidence has suggested that selenium supplementation reduces the risk of developing prostate cancer in men with normal baseline PSA (prostate specific antigen) levels, and low selenium blood levels. This is the subject of large well-designed studies, including the Nutritional Prevention of Cancer Trial (NPC), and the ongoing Selenium and Vitamin E Cancer Prevention Trial (SELECT), as well as prior population and case-control studies. The NPC was conducted in 1,312 Americans, and reported that daily selenium reduces the overall incidence of prostate cancer - although these protective effects only occurred in men with baseline PSA levels less than or equal to 4 nanograms per milliliter, and those with low baseline blood selenium levels. The NPC trial was primarily designed to measure the development of nonmelanoma skin cancers, not other types of cancers, and therefore these prostate cancer results cannot be considered definitive. To settle this question, further study is underway: The SELECT trial is in progress, with a goal to include 32,400 men with serum PSA levels less than or equal to 4 nanograms per milliliter. SELECT was started in 2001, with results expected in 2013.Laboratory studies have reported several potential mechanisms for selenium's beneficial effects in prostate cancer, including decrease in androgen receptors and PSA production, antioxidant effects, angiogenesis inhibition, or apoptosis. It is not known if selenium is helpful in men who already have been diagnosed with prostate cancer to prevent progression or recurrence of disease. It does appear that selenium may not be beneficial in those with elevated PSA levels, or with normal/high selenium levels. It remains unclear whether men at risk (or all men) should have their serum selenium values measured; results of the SELECT study may provide additional guidance. There is evidence that low selenium levels are associated with an increased risk of prostate cancer, and several mechanisms for the beneficial effects of selenium supplementation have been suggested.In the NPC trial, no benefits were seen in reducing the risk of colorectal or lung cancers. Although an overall reduction in cancer risk was observed, it is not clear which specific types of cancer besides prostate cancer prevention may benefit.

Preliminary research reports that selenium supplementation may help improve asthma symptoms. Further research is needed to confirm these results.

Early study results suggest that supplementation with selenium and other trace elements (copper, zinc) may increase the rate of burn wound healing. Additional research is necessary before a clear recommendation can be made.

Several studies suggest that low levels of selenium (measured in the blood or in tissues such as toenail clippings), may be a risk factor for developing cancer, particularly prostate cancer. Population studies suggest that people with cancer are more likely to have low selenium levels than healthy matched individuals, but in most cases it is not clear if the low selenium levels are a cause or merely a consequence of disease. It remains unclear if selenium is beneficial in the treatment of any type of cancer.

Low selenium levels have been associated with the development of cardiomyopathy, and selenium supplementation is likely of benefit in such cases (for example in Keshan disease). However, most cases of cardiomyopathy are not due to low selenium levels, and therefore selenium may not be helpful. It has been suggested that low selenium levels may be a risk for coronary heart disease, although this remains unclear.

Despite the documented antioxidant and chemopreventive properties of selenium, studies of the effects of selenium intake and supplementation on cardiovascular disease yield inconsistent findings. Better-designed trials are needed to reach a firm recommendation.

Preliminary research reports that selenium supplementation may affect the development of cataracts. Further research is needed before a clear conclusion can be drawn.

Study results of selenium supplementation during chemotherapy are mixed. General concern has been raised that antioxidants may interfere with radiation therapy or some chemotherapy agents, which themselves can depend on oxidative damage to tumor cells for anti-cancer activity. Therefore, patients undergoing cancer treatment should speak with their oncologist and pharmacist before taking selenium supplements.

Preliminary research of selenium supplementation in cystic fibrosis patients yields indeterminate results. Further research is needed in this area before a conclusion can be drawn.

Studies report that selenium-containing shampoos may help improve dandruff, and selenium is included in some commercially available products.

The benefits of selenium supplementation in dialysis patients remain unclear. Some methods of dialysis may lower plasma selenium levels.

Evidence of benefit is inconclusive in this area.

Selenium supplementation has been studied in HIV/AIDS patients, and some reports associate low selenium levels with complications such as cardiomyopathy. It remains unclear if selenium supplementation is beneficial in patients with HIV, particularly during antiretroviral therapy.

Preliminary research reports that selenium can be beneficial in the prevention of several types of infection, including recurrence of erysipelas (bacterial skin infection associated with lymphedema) or Mycoplasma pneumonia. Further research is needed to confirm these results before a clear recommendation can be made.

Preliminary research shows a decrease of symptoms of elevated intracranial pressure (headaches, nausea, vomiting, vertigo, unsteady gait, speech disorders, and seizures). More research is needed before a recommendation can be made.

Selenium supplementation has been studied in various liver disorders, including hepatitis, with mixed results.

Selenium supplementation has been studied for male infertility and sperm motility with mixed results. Evidence is lacking regarding potential effects on female infertility.

Selenium supplementation has been studied in low birth weight infants. Additional evidence is warranted in this area before a clear conclusion can be drawn.

Preliminary research reports that selenium supplementation may decrease lymphedema. Further research is needed to confirm these results before a clear recommendation can be made.

Low selenium status has been demonstrated in several malabsorptive syndromes and in some digestive and gastrointestinal allergic conditions. There is some evidence that children with food allergies have a higher risk of selenium deficiency. There is no clear benefit of selenium supplementation as a therapy for malabsorptive syndromes, although vitamin supplementation in general may be warranted.

Selenium and vitamin supplementation has been studied in myotonic dystrophy with mixed results.

There is inconclusive evidence regarding the use of selenium in pancreatitis.

Preliminary study in women with pregnancy-induced hypertension has reported reduced edema, without significant impact on birth outcomes. No clear conclusion can be drawn in the absence of additional well-designed research.

Research is inconclusive in this area.

Studies of selenium supplementation for mood elevation and quality of life yield mixed results. Further research is needed before a firm conclusion may be reached.

Selenium supplementation has been studied in rheumatoid arthritis patients with mixed results. Additional research is necessary before a clear conclusion can be drawn.

Study results of selenium supplementation in septic patients are mixed.

Supplementation with selenium may improve skin lesion status, although more studies are needed before a clear conclusion can be drawn.

Photoprotection was initially observed in preliminary research using selenium supplementation and other antioxidants, although there is some evidence of ineffectiveness in preventing light-induced erythema (skin redness).

An early toxic effect of selenium is disruption of endocrine function, including synthesis of thyroid hormones (T3). Selenium has been suggested to improve inflammatory activity in chronic autoimmune thyroiditis or Grave's disease. Further research is needed before a clear conclusion can be drawn.

Commercially available 1% selenium sulfide shampoo has been reported as equivalent to sporicidal therapy in the adjunctive treatment of tinea capitis infection, although further high quality evidence is warranted.

Preliminary study of topical selenium (selenium sulfide shampoo) is inconclusive.

Evidence from the Nutritional Prevention of Cancer (NPC) trial suggests that selenium supplementation does not significantly reduce the risk of developing colorectal cancer. This randomized study was conducted in 1,312 Americans over a 13-year period, and compared the effects of daily selenium versus placebo. Although initial (interim) analysis suggested possible benefits, a later analysis found a lack of statistical significance.

Kashin-Beck disease is an osteoarthropathy endemic in selenium- and iodine-deficient areas. Preliminary evidence suggests that selenium supplementation does not significantly improve this disease.

Evidence from the Nutritional Prevention of Cancer (NPC) trial suggests that selenium supplementation does not significantly reduce the risk of developing lung cancer. Other evidence is inconclusive.

Preliminary studies suggest that selenium supplementation is not helpful in muscular dystrophy.

Selenium-ACE, a formulation containing selenium with three vitamins, has been promoted for the treatment of arthritis. Research has failed to demonstrate significant benefits, with a possible excess of side effects compared to placebo.

Results from the Nutritional Prevention of Cancer (NPC) trial, conducted among 1,312 Americans over a 13-year period, suggest that selenium supplementation given to individuals at high risk of nonmelanoma skin cancer is ineffective at preventing basal cell carcinoma, and actually increases the risk of squamous cell carcinoma and total nonmelanoma skin cancer. Therefore, selenium supplementation should be avoided in individuals at risk or with a history of nonmelanoma skin cancer.