What are some of the possible negative side-effects to electronic inhalation of the vapor? Although they do not produce tobacco smoke, e-cigarettes still contain nicotine and other potentially harmful chemicals. Nicotine is a highly addictive drug, and recent research suggests nicotine exposure may also prime the brain to become addicted to other substances. Also, testing of some e-cigarette products found the vapor to contain known carcinogens and toxic chemicals (such as formaldehyde and acetaldehyde), as well as potentially toxic metal nanoparticles from the vaporizing mechanism. The health consequences of repeated exposure to these chemicals are not yet clear. Another worry is the refillable cartridges used by some e-cigarettes. Users may expose themselves to potentially toxic levels of nicotine when refilling them. Cartridges could also be filled with substances other than nicotine, thus possibly serving as a new and potentially dangerous way to deliver other drugs.