The problem is that skin is designed to keep things out. “It’s a barrier,” says Carol Johnston, professor of nutrition at Arizona State University. “That’s what the skin is designed to do.” For instance, if you spill something on your skin, you can simply wipe it off without it being absorbed, she explains.

(Five things you should know before trusting that supplement.)

The skin is a lipophilic barrier, meaning it is more likely to absorb fat-soluble substances over water-soluble ones. For example, vitamin C, which is water-soluble, struggles to penetrate the dermis effectively. Fat-soluble vitamins, such as A, D, E, and K, may be better candidates for delivery through vitamin patches. But “just by being fat-soluble or being in an oil doesn’t guarantee delivery into the bloodstream in a reliable way,” says David Seres, professor of medicine at the Institute of Human Nutrition at Columbia University Medical Center.

Even if these vitamins do pass through the skin, the amount on the patches and the amount absorbed is probably so minimal that their effectiveness remains questionable, says Nial Wheate, a professor at the School of Natural Sciences at Macquarie University. “Patches cannot hold anywhere near as many vitamins and minerals as a tablet can,” he says.

Some studies have examined whether these patches deliver vitamins into the bloodstream, but the results are mixed. One small study looked at people who had undergone gastric bypass surgery—a group that typically needs nutrient supplementation after their operation. Those who got a multivitamin patch instead of a supplement were more likely to have vitamin D deficiency and reduced levels of other key vitamins and minerals in their blood compared to the supplement group.