Those enticing little gummy vitamins that look more like candy than supplements are not just for kids. By some estimates, according to Time magazine, adults now make up 80% of the market. In fact, gummy multivitamins accounted for 7.5% of the $6 billion multivitamin market in the United States in a 2016 survey.

Experts blame “pill fatigue” for driving the gummy trend. Many Americans take multiple vitamins and minerals daily, and for folks who have trouble swallowing pills, gummies can make the chore easier according to a report by AARP.

But Dr. Tod Cooperman, president of ConsumedrLab.com, a private company that tests the safety and quality of consumer products, says buyers should beware. When his researchers tested gummies, they found a full 46% failed testing.

“It’s a lot harder to make a good gummy than it is to make a tablet or capsule,” he says. In a supplement analysis, ConsumerLab.com found that four out of five gummy products did not contain the exact amount of nutrients listed on their labels. And gummies were more likely to fail supplement testing than more conventional products.

“Many companies have trouble controlling the amounts of ingredients in each gummy,” says Cooperman. He says that some manufacturers therefore resort to spray coating vitamins and nutrients on the outside of the finished candy. This can cause gummies to lose potency over time.

“That leads to some manufacturers putting in a lot more of certain vitamins than listed on the label to ensure the product provides at least 100% of the label amounts throughout its shelf life.”

For some people, excessive nutrient intake may cause health problems, including an increased risk for certain types of cancer.

ConsumrLab.com also found that relatively few gummies contained iron, an essential mineral for pregnant women that helps lower the risk for preterm birth and other complications. That’s because iron has a metallic taste that would be off-putting in the candy-like supplement.

The good news is that some gummies — including kids gummies made by Flintstone Vitamins and women’s gummies made by Nature’s Way — passed CL testing.

However, Dr. Mark Moyad, the Jenkins/Pomkempner Director of Preventative and Alternative Medicine at the University of Michigan Medical Center has reservations about the whole gummy concept.

“I’m concerned about kids and adults becoming accustomed to getting nutrients in sugary forms,” he says. A typical gummy vitamin contains one to two grams of sugar while a 1,000-milligram dose of Nature Made Gummy Vitamin C has 8 grams — or the equivalent to two teaspoons — of sugar.