There are some teens out there looking to build muscle and bulk up for sports.
The supplement creatine is marketed to do just that, but it's not recommend for anyone younger than 18 years old by both the American Academy of Pediatrics and American College of Sports Medicine.
In a new study in the February issue of Pediatrics, researchers called various health food stores pretending to be a 15-year-old boy looking to increase his muscle mass.
About 67 percent of the stores recommended creatine.
"Creatine has certain side effects to a body, and especially to the body of a growing teen that are very concerning," says the study's author Dr. Ruth Milaniak.
There are no laws preventing a health food store from selling it to minors, but all bottles do come with a warning label that creatine should not be sold to those under 18.
"We have very little knowledge of what happens when a youth takes creatine," says Milaniak. "The supplement industry is not regulated, so there's a vague sense of what these side effects are."
Her biggest concern is that creatine may affect the way a child's muscles develop, so a child may see immediate gains from taking the supplement and then experience physiological problems decades later.
"You may develop muscles that are unbalanced. You may develop a bone structure or body structure that is not the way it should be and that may lead you later, when you're at the peak of your career, having muscle issues or having ligament tears because your muscles didn't develop the way they were supposed to," Milaniak says.