David Mitchener, 89, was admitted to East Surrey Hospital last May with hypercalcemia, meaning his blood was overloaded with calcium. He was treated but died in the hospital 10 days later. In December, a coroner’s investigation ruled Mitchener’s demise “death by misadventure.” This U.K. term implies his death was not only accidental but also due to an action he deliberately took. Mitchener’s alleged risky behavior? Taking too much vitamin D.

Mitchener’s cause of death was multipronged; his autopsy report named hypercalcemia, acute kidney failure, congestive heart failure, and ischemic heart disease as culprits in addition to vitamin D toxicity. Even so, Surrey assistant coroner Jonathan Stevens was concerned enough about the supplement’s role in the octogenarian’s death that he filed what’s called a report to prevent future deaths with the Courts and Tribunals Judiciary.

For at least nine months before his death, Mitchener had been taking vitamin D supplements purchased from NaturPlus UK, according to the Jan. 19 report. Several of the retailer’s products contain vitamin D, but the redacted report didn’t specify which one(s) Mitchener had been taking, how often, or in what dosage(s). Regardless, the document alleges NaturPlus UK packaging didn’t feature adequate warnings about supplement overdose.

“Vitamin supplements can have potentially very serious risks and side effects when taken in excess,” Stevens wrote. “Current food labeling requirements do not require these risks and side effects to be written [on] the packaging.”

He added, “In my opinion, action should be taken to prevent future deaths and I believe you have the power to take such action.”

The report was sent to NaturPlus UK, the Food Standards Agency, and the Department of Health and Social Care, which were required to respond by March 15.

Without vitamin D, your body can’t absorb calcium and therefore can’t build and maintain strong bones. The nutrient, also called calciferol, is critical for the health of your immune, nervous, and musculoskeletal systems, too. But because few foods naturally contain vitamin D, dietary supplements featuring it are a dime a dozen. A surplus of vitamin D, though, can be dangerous.

How much vitamin D should I take daily?

In the U.S., the amount of vitamin D in a product is typically labeled in International Units (IU) and/or micrograms (mcg). The Office of Dietary Supplements recommends daily vitamin D intakes of 600 IU (15 mcg) for adults ages 19 to 70, and 800 IU (20 mcg) for those 71 and older.

People tend to have a positive association with vitamins, so the idea they can be toxic is difficult to grasp, says Joanne Slavin, Ph.D., a professor in the Department of Food Science and Nutrition at the University of Minnesota.

“We know they can be, especially fat-soluble vitamins. Vitamin A, vitamin D are the ones that we’re most worried about,” Slavin tells Fortune, referring to them as nutrients of concern. Vitamins A, D, E, and K are fat-soluble, meaning they can dissolve in fats and oils, and are stored in your liver and fatty tissue. “If you’re taking too much over time, you can get higher levels than you need, and the body can’t really get rid of it.”

To help prevent overconsumption, the Office of Dietary Supplements established daily upper limits (ULs) of what is safe to consume. People 9 and older, for example, may have a maximum 4,000 IU (100 mcg) of vitamin D per day.

“Supplements, in the U.S. at least, are not supposed to have more than the UL in any single dose,” Slavin says. “But you could take 10 doses—we can’t control that.”

In the U.K., the Scientific Advisory Committee on Nutrition recommends 400 IU (10 mcg) per day for everyone ages 4 and older. The country follows the European Food Safety Authority’s daily UL of 4,000 IU (100 mcg) for people 11 and older. At least two NaturPlus UK supplements contain 5,000 IU (125 mcg) of vitamin D in a single capsule.

Mitchener’s vitamin D level was tested before his death. The coroner’s report showed a score of 380, “the maximum level recordable by the laboratory.” No unit of measurement was listed, but this number was presumably a blood test in nanomoles per liter (nmol/L). The U.S. Office of Dietary Supplements considers a reading above 125 nmol/L high enough to cause health problems.

In 2022, nearly 10,500 cases reported to the U.S. National Poison Data System mentioned vitamin D, up from about 5,900 in 2012. 

Because vitamin D helps regulate calcium levels in your body, hypercalcemia is a direct result of vitamin D toxicity, says Jamie Alan, Pharm.D., Ph.D., an associate professor in the Department of Pharmacology and Toxicology at Michigan State University.

“Signs of increased calcium can include weakness, fatigue, and bone pain,” Alan tells Fortune via email. “More serious symptoms can include stupor, coma, kidney stones, stomach ulcers, nausea, vomiting and diarrhea, and cardiac arrhythmias.”

While vitamin D toxicity is rare, it’s a good idea to keep tabs on your intake and keep your doctor informed of any supplements you’re taking, Alan says.

“Over-the-counter supplements can be unreliable in terms of what the supplement contains and the dose you will actually receive,” Alan says, noting some companies, such as ConsumerLab, do independent testing. “It’s worth checking out if you decide to try an OTC supplement.”

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How can I get a safe amount of vitamin D?

Vitamin D is nicknamed “the sunshine vitamin” because your body makes the nutrient when sunlight touches your bare skin. However, genetic and environmental factors, such as having dark skin or living in a smoggy area, can limit your body’s vitamin D production. Wearing sunscreen, which helps protect your skin from cancer-causing ultraviolet radiation, also hinders the process. Where you live and the time of year impacts your vitamin D conversion, too. 

“That’s why milk [is] fortified in the U.S. with vitamin D, just to make sure most people are going to get it,” Slavin tells Fortune. “Which made total sense back in the 1940s, when everybody drank two servings of milk every day, but now people don’t.”

The U.S. Department of Agriculture encourages consumption of these foods to help you get enough vitamin D in your diet: 

• Canned light tuna
• Canned sardines
• Flounder
• Fortified low- or nonfat American cheese
• Fortified orange juice
• Freshwater rainbow trout
• Herring
• Low- or nonfat milk
• Low- or nonfat plain yogurt
• Low-fat plain kefir
• Raw mushrooms
• Salmon
• Tilapia
• Unsweetened soy, rice, or almond milk

Many cereals are fortified with vitamin D, and other foods including eggs, beef liver, chicken breast, and cheddar cheese have the nutrient in small amounts, according to the Office of Dietary Supplements.

For more on safely consuming vitamin D:

• 5 foods rich in vitamin D to protect your memory and boost brain function
• Vitamin D supplements may lower your heart attack risk. Here’s how much you need each day
• Vitamin D may improve brain function and strengthen memory. Here’s how much you need each day
• Taking a vitamin D supplement may lower your risk for dementia. Here are the groups who may benefit most