A number of supplements may interact in harmful ways with prescription or over-the-counter drugs. For example, St. John's wort may alter the breakdown of many drugs including antidepressants and birth control pills. Vitamin K, ginkgo biloba, garlic, and vitamin E may interact with blood-thinners.  That's why it's essential that you consult your physician before starting a supplement regimen or making changes to your treatment regimen or prescribed medications. 

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Count nutrients, not calories. If you want to improve the quality of your diet, focus on the nutrients in your food instead of calories.  “Our energy intake can only depend on our ability to extract that energy from food,” writes blogger and nutritionist Rhiannon Mack. “All calories are not created equal – it is the quality of the calorie that has the most impact on health, not necessarily the quantity,” she explains in a post that appeared in Move Nourish Believe.
Our sedate, modern lifestyle reduces the need for energy from food, which implies a lower food intake or obesity. Loren Cordain, PhD, and coworkers have estimated that hunter-gatherers had significantly higher energy needs than the typical modern office worker. [27] A lower energy intake generally reduces the absolute intake of all nutrients, while the need for some nutrients is not always reduced proportionally with energy intake. Overall this suggests that more exercise along with a more nutritious diet, including supplements of essential nutrients and less carbohydrates, will help to prevent obesity and maintain health.
Calcium may even be harmful for men, at least in large amounts. The worry is prostate cancer, and two Harvard studies have raised the alarm. In 1998, the Health Professionals Follow-up Study found that a high consumption of calcium from food or supplements was linked to an increased risk of advanced prostate cancer. The risk was greatest in men who got more than 2,000 mg a day. More recently, the U.S. Physicians' Health Study reported that a high consumption of calcium from dairy products appeared to increase a man's risk of prostate cancer by up to 37%. A study from the Fred Hutchinson Cancer Research Center in Seattle also found a link between calcium and advanced prostate cancer.
The U.S. Food and Drug Administration does not regulate herbs and other dietary supplements in the same way it does prescription and non-prescription drugs. Unlike pharmaceutical manufacturers, who must prove a drug's safety and effectiveness prior to putting it on the market, supplement manufacturers are not required to prove the safety and effectiveness of a supplement before it is made available to consumers. (The Dietary Supplement Health and Education Act has exempted supplement manufacturers from these regulations). 

Dietary supplements are complex products. The FDA has established good manufacturing practices (GMPs) for dietary supplements to help ensure their identity, purity, strength, and composition. These GMPs are designed to prevent the inclusion of the wrong ingredient, the addition of too much or too little of an ingredient, the possibility of contamination, and the improper packaging and labeling of a product. The FDA periodically inspects facilities that manufacture dietary supplements.
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