A saleswoman helps a customer as she shops for an electronic cigarette at the Vapor Shark store in Miami.Joe Raedle | Getty ImagesSales of e-cigarettes have decelerated over the past four weeks, led by a slowdown in demand for Juul products amid a mass of negative headlines and a spate of vaping-related illnesses, according to new Nielsen data.E-cigarette sales volume rose 38.1% in the four-week period ended Sep. 21, compared with 48.1% growth in the 12-week period, according to Nielsen. Sales for products made by Juul Labs grew just 31.2%, a slowdown from its 56.2% growth for the 12-week period.And Juul’s dollar share has also declined to 66.7%, taken by competing e-cigarette companies Imperial Brands’ Blu, NJOY, Reynolds American’s Vuse and Japan Tobacco’s Logic. Juul has been in headlines as the Food and Drug Administration cracks down on vaping, slamming the company for illegally advertising its nicotine pods as a safer alternative to cigarettes and threatening to fine or seize products if the marketing isn’t corrected.Nielsen, which tracks sales at convenience stores and mass retailers but not vape shops or online, also noted that the FDA-mandated removal of flavored e-cigarette products could result in higher sales of traditional, combustible cigarette as vapers turn to the tobacco product, contradictory to the FDA’s intentions. Sales for cigarettes didn’t decline as sharply as had been the case. In the past four weeks, cigarette sales fell 6.7% compared with a 7% decease in the 12-week period.Last week, British tobacco company Imperial Brands lowered its full-year forecast, citing “considerable deterioration” in the U.S. vaping market because of increased regulatory uncertainty. Nielsen said it has yet to see that reflected in its data.