Dietary Supplement Side Effects

Are you one of the more than 70% of Americans that take a dietary supplement regularly? Then you should be up-to-date on all the latest supplement science! Here, Ana, one of our nutritional scientists, breaks down all the dietary supplement side effects, dangers, and benefits to help keep you and your supplementing family safe!

Since 1994, when the Dietary Supplement Health and Education Act (DSHEA) gave the FDA authority to regulate dietary supplements in the United States, the market has expanded enormously!  That year, there were roughly 4,000 dietary supplement products on the US market. By 2014, there were more than 85,000.  

What’s more, by 2012 over one-half of the Americans reported using some kind of dietary supplement!  And the latest data we have (from 2018) says that 69% of people between 18-34 and 78% of older adults use dietary supplements.


I find these numbers very, very worrisome!  

After all, does it sound plausible to you that more than ⅔ of the population really needs dietary supplements?  

And if they aren’t needed, what’s driving so many people to the supplement aisle?  

I have been super excited about this topic for a long time.  And I am looking forward to sharing all the science surrounding safe and unsafe supplement use with you!

What are Dietary Supplements?

Dietary supplements

Dietary supplements usually come in the form of:

  • tablets
  • pills
  • capsules
  • extracts
  • liquids

They contain concentrated compounds intended to improve people’s dietary or nutritional status.  

Dietary supplements you can find on the market include:

  • amino acids
  • vitamins
  • minerals
  • multivitamins
  • herbal supplements
  • (among many others)

Unlike medications, supplements are not supposed to treat, diagnose, prevent, or cure diseases.  That’s why you can’t find statements such as “reduces pain” or “treats heart disease” on their packaging.  These claims can only be used on drug packaging and you shouldn’t see supplements as a way to treat diseases.

What’s more, you shouldn’t see supplements as a substitute for food.  You can’t replace the nutrients you should get from food with nutrients from supplements. Supplements are only supposed to be a “support” or “boost” for your nutrition.  They can help keep you healthy when added to a well-balanced, diverse diet.  

I want to emphasize this point because this is a common misconception.  People believe they can keep their lifestyle as it is and just take a pill to make them healthy.  

And look, I know that changing your life from the roots is time-consuming and requires a lot of effort.  (It took me years before I could say with complete honesty that I follow a healthy diet!) Taking the pill is so much easier!  

But eating a healthy diet is much better for your health in the long run.

I’ll do my best in the rest of this article to explain why!

Are There Benefits to Taking Dietary Supplements?

Supplements can be helpful if, as I stressed already, you need them.   

Thanks to the modern scientific tools we have today, we actually know which people have a high risk for vitamin or mineral deficiencies.  These are people who may need supplements.

According to the current recommendations, these group-nutrient pairs are:

  • People over the age of 50 – who should supplement their diet with vitamin D, B12, and folate to maintain bone and brain health.
  • Women of childbearing age – (even when they aren’t pregnant) who should take folic acid, vitamin D, and iron.  Lack of these nutrients can have health risks for both baby and mom. Insufficient amounts of folic acid may increase the risk of neural tube defects. Vitamin D deficiencies may lead to impaired skeletal development.  And a lack of iron may lead to anemia.  
  • Children under age 5 – need lots of vitamin A, C and D.  Kids who follow a diverse and healthy diet do not need supplements.  But if you have a picky eater on your hands, supplements may be necessary.
  • People who cover their skin – who are at risk of vitamin D deficiency and may benefit from supplements or increasing vitamin-D-rich foods in their diets.
  • Vegans – who avoid animal-foods completely, need support from supplements in order to get sufficient amounts of vitamin B12 and vitamin D3.
Mom giving child a piggyback ride in the sunshine

Who Else Can Benefit From Supplements?

To answer this question, researchers turn to meta-analyses.  

Meta-analyses look at all the evidence about a single supplement.  They pool the data from similar trials to see what the total effect of a supplement is on a large number of people.

Luckily, some meta-analyses have found certain supplements to be effective!  That’s the case with B-vitamins, for example.

One network meta-analysis summarized and analyzed the data from 17 trials (including a total of 86,393 patients) who consumed either B-vitamin supplements, placebo, or no intervention at all.  The study found that consuming vitamin supplements is linked with a reduced risk of having a stroke.

Another meta-analysis found that folic acid supplementation during pregnancy may prevent women from developing preeclampsia.

But meta-analyses also sometimes show that supplements aren’t helpful.  

For example, for decades, doctors recommended vitamin D and calcium supplements for older adults to prevent fractures.  The latest meta-analysis, however, found no evidence that these supplements decreased the risk of breaking a bone.  

And this is not the only vitamin that has failed in meta-analyses.  But I’ll dig into more detail about that in a moment!

Why are Dietary Supplements Riskier than Food?

Close up of freshly picked apples

Think about it this way:

What would l happen if you were craving sugars, so you ate a whole pile of apples?  Well, you’d likely end up with stomach cramps, gas, and bloating. This would probably be a short-lasting experience, with your symptoms going away after your first visit the toilet. But, after that first apple overdose, you would definitely think twice about doing it again! 

“Over-consuming” nutrients through foods is less risky than taking to many supplements because you are far less likely to be able to consume as much of them without your body telling you to back off!  

Additionally, the absorption rate of nutrients is different between foods and supplements. Absorption of nutrients is much slower from food than from a supplement because the nutrients are trapped in the food matrix.  This means your body has more time to get rid of any extra nutrients you might consume from food, making them less likely to harm you!

But if you overdo it on supplements, all the nutrients they contain are quickly absorbed. You can take too much pretty easily without having time for your body to notice.

And you might even do it by mistake.

This is because supplement labels are often incorrect. There have been tons of reports against manufacturers because their supplements contained several times larger doses than their labels claimed.

Add to this the fact that many people take larger doses of supplements than the label recommends because they think it will “help more” and you can see there is a serious risk here!

Are Dietary Supplements Riskier than Prescription Drugs?

The American Cancer Society offers a very good description of the difference between drugs and supplements:

  • Drugs are considered unsafe until proven safe.”
  • Dietary supplements are considered safe until proven unsafe.”

So, when a new drug is made, it needs to go through many carefully designed human clinical trials before the FDA says: “Ok, we now have sufficient amount of data to know that this drug is safe and really helps treat a disease.”  

Only then do they release it onto the market.  And when they do, they include a detailed label with all the health information about the drug, including side effects and drug-drug interactions.  

Supplements, on the other hand, are released onto the market with no testing.  They are considered safe until someone reports an adverse reaction to it!

The responsibility of making sure a supplement is safe falls solely on the manufacturer.  The FDA only gets involved if there is lots of evidence the supplement is harming people’s health.  

Some manufacturers definitely take advantage of the freedom these regulations give them. There have been all kinds of scandals with dietary supplements:

  • not being safe
  • not containing the ingredients they claim to
  • containing very different dosages than listed on the label
  • swapping parts of plants (roots for leaves, for example) listed on the label, making the nutrient dosages incorrect.

One scandal that got a lot of attention in the was a group of herbal supplements which were found to contain ephedrine, a dangerous compound that the FDA banned in the US.

And to make all of this worse, supplements are sold over-the-counter.  This means you don’t need a prescription and you don’t have to talk to your doctor about risks or known dietary supplement side effects before you pick one up!

What Are Potential Dietary Supplement Side Effects?

Any type of imbalance in your diet or lifestyle can harm your health.  But consuming megadoses of supplements over long periods of time can have very serious consequences for your health, indeed.  

This is for several reasons:

  1. There is an upper limit for micronutrients in your body; having levels above this can harm your health;
  2. All micronutrients interact with each other, so taking megadoses of one nutrient can harm the metabolism of all your micronutrients;
  3. Supplements interact with drugs (even the natural herbal ones!), which may make your medication or your supplement work too well or not enough.

So, what, specifically, can happen if you overdose with supplements?  

Typically, the short-term side effects are mostly gastrointestinal problems.  In the case of high doses of vitamin C or magnesium, for example, people report diarrhea.  Too much iron, on the other hand, is likely to cause constipation, nausea, vomiting, and stomach pain.

These are all symptoms that usually don’t last long!  But there are some cases where dietary supplements side effects become considerably more serious — namely, if you to take far too much for a long period of time.

What are the Long-Term Side Effects of Dietary Supplements?

Long-term overdoses can lead to more serious dietary supplement side effects. The exact effects, however, depend on the type of the nutrient your body gets loaded with.  Luckily, we now have lots of data that tell us what those risks are and where should we be cautious.

Below, I listed some of the groups of supplements that people commonly use today and what the science says may happen if you overdo it with them!

Vitamins and Minerals

Water-soluble vitamins (B-vitamins and vitamin C) are very rarely reported to cause overdoses.  This is because these vitamins can get in your urine and you can literally pee out any extra.  

Fat-soluble vitamins, on the other hand, pose a real risk for overdose.  

The symptoms of an overdose depend on the vitamin:

Vitamin E

Very high doses of vitamin E (800-1,200 mg/d) can lead to bleeding because vitamin E thins your blood.  Doses higher than 1,200 mg/d may result in diarrhea, weakness, and blurred vision.

In patients with head and back cancer, vitamin E supplements may increase the risk of cancer returning after remission.

Vitamin A

Vitamin A and its other forms have been shown to be toxic as well.  Two large clinical trials found that male smokers who received 30 and 20 mg of beta-carotene daily had a significantly greater risk of lung cancer.  Long-term high doses of vitamin A are also linked with poor bone health, low bone mineral density, and a high risk of fracture.

Vitamin D

One of the most common side effects of large doses of vitamin D is high blood calcium levels.  This can affect your bones, heart, and kidney.  Apart from a case report of a 70-year-old man with extremely high levels of vitamin D and calcium and kidney disease, a study from the Kashmir region in India reported 62 cases of vitamin D overdose.  People experienced a range of symptoms including nausea, vomiting, and kidney failure.

Related: Building a Healthy Diet and Lifestyle

Some minerals can also pose risks at higher doses, especially iron.


People who consume excess amounts of iron or multimineral supplements may have an increased risk of iron storage diseases, which can result in liver damage.  This risk may be even higher for those who drink alcohol while taking iron supplements.

Fish Oil and Omega-3 Fatty Acids

Back in 1971, one study found that Eskimos (living in Greenland) had much better lipid profiles and lower rates of ischemic heart disease than both Danes and Greenlanders living in Denmark.  What was the catch?  They were eating tons of fatty fish!  Since then the science and the public have been adoring fish!  

Research eventually found that many of the health effects of fish came from a group of healthy fats called omega-3 fats.  Today, you can consume these fatty acids as supplements!  

Although you can find conflicting results in the scientific studies, it seems that these supplements are quite safe (you can consume up to 2,000 mg/d with minimum risk of side effects).  The only time problems may occur is if you consume fish liver oil, rather than purified omega 3 fats.  Fish liver oil contains large amounts of vitamin A so you could develop a vitamin A overdose if you take it regularly.

Herbal Supplements

Herbal medicine is the precursor of modern medicine.  Ancient peoples used different herbal mixtures and tinctures to treat their diseases, sometimes pretty effectively.  

Unfortunately, the long history of using certain plants and herbs as medicine makes some people believe they are definitely safe.  

Often, these people don’t even report if they are taking a herbal supplement to their doctors because they don’t think it matters. (It is completely natural, after all.  What could go wrong?).

This, together with the how hard it is to know what’s really in supplements (see above), monitoring side effects from herbal supplements is very difficult and they are rarely reported.   

In most cases, when people report herbal dietary supplement side effects, they are mild and include things such as fatigue, nausea, headache.  But they can be worse.

For example:

  • Garlic and ginkgo biloba extracts have been reported to cause excessive bleeding.  
  • In several case studies, echinacea was reported to cause acute liver failure.  
  • Valerian (a common anti-anxiety herb) has been reported to cause jaundice and acute liver failure.

Do Dietary Supplements Even Work?

To answer this question, I again dove into the published meta-analyses. Actually, I was surprised how many of them there were to read through!  The data mostly looked at people diagnosed with some type of disease, but some did look at healthy individuals.  

Below are some of the conclusions derived from these studies.

Antioxidant Supplements

When meta-analyses looked into the impact of antioxidant supplements (mostly vitamins A, C, E, and selenium), they found no evidence they could be helpful for:

What is worse, beta-carotene, vitamin E, and vitamin A supplements appeared to increase the overall risk of death.

Fish Oil and Omega-3 Fatty Acids

Although current guidelines strongly support the use of fish omega-3 fats to protect against heart disease, not all individual trials reported this effect.  

That’s why a large meta-analysis was done.  It concluded that supplementing with omega-3 fatty acids may lower the risk of cardiac death but not any other cardiovascular health outcomes (having a non-fatal heart attack, having a stroke, etc.).

Learn More: How to Eat Well and Be Happy

Herbal Supplements

Among the huge variety of herbal supplements present at the market, there are only a few that have been clinically tested:

  • Asian ginseng – Meta-analyses support using Korean ginseng supplements to lower blood pressure in individuals who are either at risk or already have high blood pressure. There is not enough evidence to know if ginseng may be able to reduce fatigue or enhance physical performance.  
  • Valerian root – Meta-analyses suggest that valerian root may be useful in reducing insomnia symptoms.  Though, most studies looked at people’s reports of how well the slept — not actually testing how much sleep they got.  So, this data should still be taken with a grain of salt.

Here, I only included studies that showed some effect of the dietary supplement.  I also wanted to point out that there are numerous other review studies that couldn’t make any conclusions at all because the evidence they had to work with wasn’t strong enough one way or the other.

I don’t want to imply that no other supplements have been found effective in studies (I mentioned some of them in the paragraph “Are There Benefits to Taking Dietary Supplements?”). The truth is, though, there are many who do not do so.  

What About Taking Supplements and Medications Together?

Let’s first review some worrisome statistics, again:

  • One in six people that are on medications uses dietary supplements as well.  
  • Two out of three patients that take both prescription medications and supplements do not tell their physician about their dietary supplement use, probably because they don’t think it matters.

If you are in those “two out of three patients”, please don’t stay there!  Talk to your doctor asap! Because supplements can interact with drugs!

They do it by:

  • Changing how your body metabolizes, excretes, or absorbs the other pill (meaning your dose of the supplement or medication is suddenly either too high or too low!)
  • Changing the ability of your medication or supplement to actually interact with your body’s cells.

As a result, the effects of the supplement or the drug may be too intense or too weak, potentially making you sick.  Here I listed some of the most common potential harmful drug-supplement combinations to avoid.

Potentially Dangerous Drug-Supplement Combinations

If you are on warfarin therapy, there is a risk that you may experience bleeding problems if, at the same time, you also take:

  • Cranberry juice
  • Fish oil
  • Gingko (you should avoid this supplement with aspirin as well)
  • Ginseng
  • St. John’s Wort
  • Vitamin E (more than 400 IU/d).

If you are taking a heart medication, you should avoid the following combinations:

  • Eleuthero (Eleutherococcus senticosus) and digoxin
  • St. John’s wort and digoxin
  • Verapamil and statins

If you are experiencing mental health issues and are taking psychiatric medications, don’t use these combinations at the same time:

  • Gingko and atypical antidepressants (such as trazodone)
  • Ginseng and monoamine oxidase inhibitors
  • St. John’s Wort and selective serotonin reuptake inhibitors, benzodiazepines, or tricyclic antidepressants

If you are interested in more details about supplement-drug interactions, you can check out this list compiled by the National Center for Complementary and Integrative Health.

Are There Any Dietary Supplements Without Side Effects?

After having said all above, I personally don’t believe there are any supplements are completely without the risk of side effects.  

I say this specifically because of the existing manufacturing regulation and how often we know the labels don’t match the products.  Since you can’t know for sure how much of a given nutrient you’re taking, there is no science that can assure you that the supplement is safe, right?  

Unfortunately, you can’t believe 100% in any label or even to any random doctor or other qualified personnel (on TV or YouTube, for example) when making these choices because they may have their own personal interest involved.  That’s why you really need to advocate for yourself. Do the research yourself and make sure you are picking the safest choice you can.

I found this easy-to-follow guide from the American Cancer Society, which I think can help you a lot when choosing a dietary supplement (if you, eventually, decide you really need one).

Take Home Message

Supplements shouldn’t be a part of your everyday life if you are healthy and following a nutritious, diverse diet.  If you though think you would benefit from taking one, consult your doctor about the right type and dosage of the supplement.

Be aware of the fact that supplements are not drugs. Thus, they are not tested before they reach the market.  That is why some manufacturers, unfortunately, make products containing either much higher doses than they claim or include ingredients (even banned ingredients) that they don’t list on the label.

Taking excessive doses of nutrients (even nutrients properly listed on labels) can be bad for your health! And even healthy doses can interact with medications if you are taking the wrong combinations.

Read Next: Food vs. Supplements — A Nutritional Anthropologist’s Take


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