Some people like to talk about all the supplements they take every day as if it's a healthy thing. But all those pills make the liver work extra hard to metabolize and detoxify the ingredients (virtually every supplement contains contaminants—such as heavy metals or silicon dioxide, which is sand—even if it's just small amounts).
They may not be treated like prescription drugs, but supplements can still hurt you (for example, yes, you can get too much vitamin D). A few well-done studies have shown that too much selenium (200 micrograms a day) has been associated with an increased risk of type 2 diabetes, aggressive prostate cancer, and skin cancer recurrence, and with no benefit for heart health. Selenium is a wonderful and probably disease-preventing nutrient; however, either too little or too much can be harmful. Here are some other ways you can OD on supplements.
• Too much vitamin A = liver toxicity
• Too much calcium, vitamin D, or vitamin C = kidney stones
• Too much red yeast rice extract = muscle pain, liver toxicity
• Too much 5-HTP or St. John's wort = nausea
• Too much vitamin B6 = sensory peripheral neuropathy
• Too much fish oil = gastrointestinal upset and an increase in bad cholesterol (LDL)
• Too much zinc = loss of taste and smell
• Too much iron = constipation
• Too much magnesium = diarrhea
• Too much iodine = thyroid problems
• Too much L-arginine = unsafe drop in blood pressure.
I could go on with every supplement ever invented, but I think you get the point. I always recommend taking the lowest dosage possible in every case.