[...] a new study from Stanford University suggests Juul's 59 mg/mL (5.9% by volume) nicotine products may have lead other e-cigarette companies to increase their nicotine content, starting a "nicotine arms race."

"Following Juul's lead, many purveyors of nicotine salt-based e-liquids offer nicotine concentrations at the 5%, 6%, and even 7% [per volume] level," Dr. Robert Jackler, lead author of the study and a professor of head and neck surgery at Stanford University, wrote.