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Use of electronic cigarettes has dramatically increased in the United States since 2010, with a forecasted growth of 37% between 2014 and 2019. There is little research on e-liquid nicotine concentration from domestic manufacturers. However, limited research outside of the United States found wide inconsistencies between the labeled concentration of nicotine in e-liquids and the actual nicotine concentration.The 7 most popular online manufacturers or distributors were identified. E-liquid samples of the 5 most popular flavors from each manufacturer were purchased in nicotine concentrations of 0 and 18 mg/mL. Of the samples purchased (n = 70), all were labeled as produced in the United States of America. The researchers anonymized the samples before sending them to an independent university laboratory for testing.The 35 e-liquid samples labeled 18 mg/mL nicotine measured between 11.6 and 27.4 mg/mL (M = 18.7, SD = 3.3) nicotine. The labeled 18 mg/mL samples measured as little as 35% less nicotine and as much as 52% greater nicotine. In the 35 samples labeled 0 mg/mL, nicotine was detected (>0.01 mg/mL) in 91.4% of the samples (range 0–23.9 mg/mL; M = 2.9, SD = 7.2). Six samples from 2 manufacturers labeled as 0 mg/mL were found to contain nicotine in amounts ranging from 5.7 to 23.9 mg/mL.This study demonstrates the nicotine labeling inaccuracies present in current e-liquid solutions produced in the United States. Incorrect labeling poses a significant risk to consumers and supports the recent regulation changes enacted by the US Food and Drug Administration. Additional routine testing of nicotine concentrations should be conducted to evaluate the effectiveness of the regulations on future e-liquid production.