In general, plasma fasting glucose and triacylglycerol levels may increase insulin activity decrease, and several indices of cell-mediated immunity decrease. Supplementation may cause anxiety, headaches and hypertension. Should be avoided by people with phenylketonuria and pregnant or lactating women. Although has been shown to be mutagenic to mammalian cells in vitro, this has not been confirmed in vitro and only one in vivo study showed this activity. This study showed an increase in bladder cancer in a mouse strain known to have an extremely high rate of spontaneous bladder cancer.
Recent research suggests a tumor suppressor effect.
Toxic dosages may be associated with hair loss, thickened fragile fingernails, muscle discomfort, dermatitis, nausea, garlic breath odor, fatigue and suppression of phagocytic and natural killer cell function. The selenite form generates free radicals by interacting with glutathione in the presence of oxygen. This form should not be used although sodium selenate is less likely to be toxic due to more rapid excretion in the urine than selenite. An adequate ascorbic acid status is important for selenium metabolism. Vitamin E and selenium have closely related mechanisms of action and a deficiency of one can be treated with either nutrient.