Selenium is safe for most adults when used over the short-term in the recommended amounts. Taking more than 400 µg of selenium can be toxic. Side effects of selenium toxicity may include nausea, vomiting, loss of energy, a metallic taste in the mouth, sleepiness, weight loss, irritability, tremor, hair loss, and muscle soreness.
While short term use of selenium at the appropriate dose appears to be safe for most adults and children, long term use is not recommended. Using selenium for a long period of time can actually increase your risk of getting type 2 diabetes.
Selenium has been shown to be safe for pregnant and breast-feeding women when it is used for a short period of time. The recommended daily amount for pregnant women is 60 µg per day and 70 µg per day for women who are breast-feeding.
It is not safe to use large amounts of selenium (i.e., more than 400 µg per day).
There appears to be a link between selenium supplements and an increase in skin cancer. However, studies are conflicting and more evidence is needed. People with a history of non-melanoma skin cancer (a common type of cancer that is usually treated successfully when it is found early) should consult a doctor before taking selenium supplements.
There have been many studies on the effects of selenium on cancer risk. Overall, the studies showed inconsistent results. Lung cancer risk does not appear to be lowered when dietary selenium is increased. However, for people who are selenium deficient (i.e., do not have enough selenium in their body), taking a supplement may slightly reduce their risk of getting lung cancer.
Selenium does not appear to reduce the risk of getting the following types of cancer:
- colorectal cancer (i.e., cancer of the colon, rectum, or bowel)
- esophageal cancer (i.e., cancer of the esophagus)
- gastric cancer (i.e., cancer of the stomach)
- prostate cancer risk
- skin cancer
Some herbs or natural health products that have anticoagulant (i.e., blood-thinning) effects may increase the risk of bleeding in people who are taking selenium.
Some species of astragalus, a plant that tends to contain large amounts of selenium, can cause toxicity if taken with selenium supplements.
There may be an interaction between the following medications and selenium supplements:
Because of its blood-thinning effects, people should avoid taking selenium supplements at least 2 weeks before surgical procedures. Taking selenium close to the time of surgery may lead to excessive bleeding.
People undergoing dialysis may benefit from selenium supplements, since dialysis can cause deficiency.
In cases of selenium deficiency caused by hypothyroidism (a condition where the thyroid gland is not functioning properly), selenium supplements should only be taken in combination with iodine. Taking selenium supplements alone can worsen hypothyroidism.
High selenium intake has been shown to reduce sperm motility, which could decrease male fertility.
Before taking any new medications, including natural health products, speak to your physician, pharmacist, or other health care provider. Tell your health care provider about any natural health products you may be taking.