The liquid itself used to fill and refill e-cigarettes may contain nicotine, THC, CBD oils, artificial flavorings, and other liquids/chemicals depending upon who you purchase it from.  A study published by the Journal of Pediatrics reported that they found numerous carcinogens such as formaldehyde, acrolein, and toluene in vape juice.  Many companies manufacture vape juice fills and refills and thus, each may vary in the formula.