Dietary Supplements: What Is Safe?

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Common misconceptions about dietary supplements

Megadosing: The “More is better” myth

Many people wonder why dietary supplements like vitamins, herbs, and botanicals are sold without a prescription from a doctor, while medicines (or drugs) are closely regulated and controlled. People often make the mistake of assuming that because supplements are sold over the counter, they are completely safe to take, even in high doses.

In the 1990s there was a trend of “megadosing” antioxidants like vitamin C, beta carotene, and vitamin E. Even though no scientific studies have ever proven that large doses of vitamin C can prevent or cure colds, many people still think this is true. Even now, you may hear claims about other benefits of taking large doses of certain vitamins. But using large doses of vitamins to fight disease in humans is not supported by scientific evidence so far.

In fact, large doses of some vitamins or minerals have been shown to be dangerous and even toxic. For example, too much vitamin C can interfere with the body’s ability to absorb copper, a metal that’s needed by the body. Too much phosphorous can inhibit the body’s absorption of calcium. The body cannot get rid of large doses of vitamins A, D, and K and these can reach toxic levels when too much is taken.

Talk with your doctor before taking large doses of any vitamin, mineral, or other supplement. Your nurse or pharmacist may also be able to give you more information on safe dosages. Even when vitamin doses are not high enough to cause toxic effects, they can have a bad impact on overall health. For instance, several large studies have found that, on average, people taking vitamin E supplements lived no longer than those who didn’t. Some even died sooner, particularly of heart failure.

The “Natural is safe” and “Natural is better” myths

In today’s world, you won’t find much support for the idea that a man-made or refined substance is better or safer than one sold in its unrefined, natural state. But supplements that claim to be “all natural” are not always better or safer than refined or manufactured substances.

Keep in mind that some of the most toxic substances in the world occur naturally. Poison mushrooms, for example, are completely natural but not safe or helpful to humans. Many plants in nature are toxic or deadly if taken internally.

Botanical supplements (such as garlic, ginger, ginkgo biloba, echinacea, and others) are made of plant material, so many of them are sold as “natural” products. But plants are made up of many chemicals. Some of these chemicals can be helpful while others are poisonous or can cause allergies in humans. Botanicals that are marketed as “all natural” are not always the most helpful ones, since they may not be refined to remove potentially harmful chemicals.

Botanicals can contain any or all parts of the plant, including roots, stems, flowers, leaves, pollen, and juices. Different parts of plants can have very different effects on humans. For instance, dandelion root is a laxative (it causes bowel movements), while dandelion leaves contain a diuretic (a chemical that increases urination). If you decide to use a botanical supplement, make sure you know what parts of the plant were put into it. If you’re unsure, contact the company and ask them how they make their supplement.

Remember, too, that safety and dose are related. The leaves or roots of many plants can be safely taken in small amounts as an herb. But concentrated extracts sold as liquids or pills may contain the plant’s chemicals in far greater amounts and may not be safe.

The “It’s been used for thousands of years, so it must work” myth

Knowing that a botanical has been used in folk or traditional medicine for thousands of years is helpful, but is not convincing proof that it works or that it’s safe. If small amounts of a plant caused painful or life-threatening side effects right away, it probably wouldn’t have been used in folk medicine or traditional medical systems. But traditional medical systems thousands or even hundreds of years ago did not have the scientific methods to detect long-term side effects. So, if a plant seemed useful over the short term but actually increased the risk of chronic disease (like cancer, heart failure, or kidney failure) after years of use, those side effects would not have been noticed. Also, if a patient’s problem got worse after using an herb, the worsening may not have ever been linked to the treatment itself. Deaths weren’t unusual; unlike today, people of all ages died of illnesses that can now be prevented, treated, or cured. Finally, in some traditional systems, herbs were given to cause vomiting or diarrhea. These effects may have been considered helpful at the time, even if the final, long-term outcome wasn’t good.

It also helps to find out whether a plant used today is being used like it was traditionally. For example, tea prepared from a certain plant might have been safely used in traditional Chinese medicine to treat occasional bouts of asthma when given by an experienced practitioner. On the other hand, daily use of much higher doses taken in a concentrated pill form with no expert supervision might be quite unsafe.

As you consider ancient treatments, remember that most herbs, plants, and other methods were used in traditional medicine systems to reduce symptoms or make the person feel better. This was helpful to people who were likely to recover anyway. Still, it was understood that death was a possible outcome of most serious illnesses. It’s safe to say that science and technology have helped us better understand the causes of illness today than anyone did centuries ago. Now, most people whose families once used these traditional healing methods prefer to be treated with modern medicine, if there’s a proven treatment available.

The “It can’t hurt to take supplements along with my regular medicines” myth

Many people assume that dietary supplements are always safe to take along with prescription drugs. This is not true. For example, certain botanicals can block or speed up the body’s absorption of some prescription drugs. This can cause the person to have too much or too little of the prescribed drug in their bloodstream. Most drug companies and producers of herbal supplements do not research possible drug interactions, so the risks of taking supplements with other drugs are largely unknown.

Talk with your health care team about any supplements you are taking or wish to take. Your doctor or pharmacist can tell you about any known interactions with medicines you may be taking. Keep in mind that with new drugs and supplements, interactions may not be known.

“The FDA wouldn’t let them make that claim if it wasn’t true” myth, and the “If it could hurt you they wouldn’t be allowed to sell it” myth

Because of the way dietary supplements are regulated, the FDA (US Food and Drug Administration) cannot check every claim made about a supplement. And, safety is up to the manufacturer. The FDA is allowed to step in only if they are aware of a problem. This is discussed in more detail below.


Last Medical Review: 11/22/2013
Last Revised: 06/02/2014