The U.S. Food and Drug Administration said Monday it has issued a warning letter to Juul Labs Inc., saying it determined the vaping company adulterated its products by selling them as modified risk tobacco products without an FDA order that permits the sale. The letter states the company engaged in labeling, advertising and/or other activities directed at consumers, including a presentation given to youth at school. "Regardless of where products like e-cigarettes fall on the continuum of tobacco product risk, the law is clear that, before marketing tobacco products for reduced risk, companies must demonstrate with scientific evidence that their product does in fact post less risk or is less harmful," the FDA said. The warning letter comes after hundreds of cases of severe respiratory diseases possibly associated with e-cigarette use. Shares of Altria Group Inc. /zigman2/quotes/208895754/composite MO -0.96% , which announced in December 2018 that it paid $12.8 billion for a 35% stake in Juul, edged up 0.6% in morning trading. It has lost 14.1% over the past three months, while the S&P 500 has gained 3.9%.