• Dietary supplements have become popular among Americans.
  • But too much of supplements, including fish oil and vitamin D, can be harmful. 
  • A toxicologist shared the side effects of overdoing some of the most popular supplements.

Dietary supplements are big business. Surveys suggest that over half of US adults take them, and the market is projected to be worth $200 billion by 2025.

But although we may think supplements are safe, even the most popular ones can be dangerous if we take too much or they interact with other medications, professor Rob Chilcott, the head of toxicology at the University of Hertfordshire, UK, told Business Insider.

What's more, supplements aren't FDA-approved like prescription drugs, so there is a risk they could be contaminated by or cut with other substances not mentioned on the packaging. One 2023 study, for instance, found that 89% of the 57 dietary supplements tested didn't accurately list the ingredients they contained on their labels.

Generally, experts agree it's best to get nutrients from food, but supplements can be useful for people with certain deficiencies or whose diets are lacking.

Chilcott shared the long-term risks of taking too much of some of the most popular supplements.

Magnesium

Magnesium is essential for muscle and nerve function, and regulating blood sugar levels. The recommended daily amount for adults is between 310 and 420 milligrams depending on age and sex, or the equivalent of eating a large handful of almonds or eight small potatoes.

Overdosing on magnesium, which the Office of Dietary Supplements said has been observed in those who have taken more than 5,000 mg in a day, can cause sleepiness, loss of reflexes, facial flushing, and, in extreme cases, paralysis and death, according to Chilcott.

Consuming too much magnesium can also lead to extremely low calcium levels, called hypocalcemia, which Chilcott said can "cause a whole host of health issues." These include depression, seizures, and arrhythmia, according to Cleveland Clinic.

Chilcott said that people who have poor kidney function or hypothyroidism are particularly at risk of magnesium overdose.

Vitamin C

Vitamin C helps the immune system function and the body absorb iron. Chilcott said it is generally considered safe and that vitamin C overdose is extremely rare. Adults should consume 75-90 mg daily, and can take up to 2,000 mg of vitamin C each day before they will experience negative health effects — which is about the same amount you'd get from eating 28 oranges or 21 bell peppers.

Chilcott said that taking more than 2,000mg of vitamin C per day can cause tiredness, kidney problems, and vitamin B12 deficiencies.

He said that people with gout, cirrhosis, and certain kidney diseases should particularly avoid high doses of vitamin C.

Vitamin D

The body needs vitamin D for the immune system, muscles, and nerves to work, and it's also important for strong bones. People are advised to consume 15 micrograms of vitamin D per day and no more than 100 mcg, according to the ODS, which is about the same amount as you would get from 13.5 eggs or five cups of fortified dairy or plant-based milk.

You're unlikely to get symptoms from taking too much vitamin D if you're taking less than 250 mcg a day, according to the ODS.

Of all the supplements Chilcott spoke about, he said this was the one that's the most potentially risky, but only at very high doses. "The known health benefits of taking a recommended dose far outweigh the risks," he said.

According to Chilcott, consuming too much vitamin D is dangerous because it increases the levels of calcium in the blood, which can lead to conjunctivitis, pain, fever and chills, vomiting, and weight loss.

The main condition that increases susceptibility to vitamin D toxicity is kidney disease, he said.

BI previously reported on a man who died from vitamin D toxicity.

Fish Oils

The omega-3 fatty acids found in foods including fish are important for cell health and have many functions in the heart, blood vessels, lungs, immune system, and endocrine system, according to the ODS.

The US Food and Drug Administration says it is safe to take up to 2 grams of fish oil supplements per day, equivalent to eating one three-ounce serving of cooked farmed salmon.

Chilcott said that the main risk associated with fish oil is the pollutants that the fish may have consumed while they were alive, rather than the supplements themselves. These toxins can include heavy metals such as mercury, lead, and cadmium.

The ODS says that methyl mercury, a heavy metal, is removed from fish oil supplements during processing and purification, and one 2021 study found that the supplements they tested didn't contain arsenic, cadmium, copper, iron, or mercury. However, the same study found a few of the supplements did contain lead.