Sarah Kavanagh and her little brother were looking forward to the bottles of Gatorade they had put in the refrigerator after playing outdoors one hot, humid afternoon last month in Hattiesburg, Miss.

But before she took a sip, Sarah, a dedicated vegetarian, did what she often does and checked the label to make sure no animal products were in the drink. One ingredient, brominated vegetable oil, caught her eye.

“I knew it probably wasn’t from an animal because it had vegetable in the name, but I still wanted to know what it was, so I Googled it,” Ms. Kavanagh said. “A page popped up with a long list of possible side effects, including neurological disorders and altered thyroid hormones. I didn’t expect that.”